At the time of the dinosaurs, the oceans were teeming with life. Below the waves, giant marine reptiles, such as the fearsome 4m (13ft) long mosasaurs, were the undisputed apex predators.

In artistic reconstructions of these ancient oceans, cephalopods – the animal group that includes squid, cuttlefish, octopuses, and their ancestors – are almost always portrayed as prey, often seen desperately swimming away from the jaws of a marine reptile to avoid becoming lunch.

However, a remarkable new fossil suggests our view of the ancient oceans is incomplete, and that giant octopuses, perhaps reaching as long as 19m (62ft), may have been the ones doing the hunting.

The fossil in question is a giant octopus jaw, belonging to a new species called Nanaimoteuthis haggarti. It is found in Late Cretaceous rocks of Japan, making it between 100 million and 72 million years old.

Like other cephalopods, octopuses have a hard beak that looks like a parrot’s bill, used to bite and tear prey, and this fossil example is enormous – larger than that of the famous giant squid Architeuthis.

Based on the shape and size of the beak, Shin Ikegami, from Hokkaido University, Japan, and colleagues, identify it as belonging to the Cirrata, a group of finned octopuses still found today in the deepest oceans. They estimate that the animal may have reached between seven and 19 metres in length. Details have been published in the journal Science.

If that upper estimate is even close to correct, Nanaimoteuthis, would represent the largest invertebrate yet described from the fossil record — an animal rivalling the largest marine reptiles in scale.

The authors also use the wear and damage on the octopus beak as indicators of ancient behaviour. Scratches and pits on the surface point to an animal hunting and crushing prey with bones or shells, not scavenging or feeding on soft-bodied organisms.

Additionally, the wear pattern is asymmetric, interpreted by the authors as evidence of a preference for chewing on one side over the other, a trait associated with higher cognitive function.

Far from being food, Nanaimoteuthis may have been one of the most formidable predators in its ecosystem, in an era we have long assumed was defined by vertebrate dominance.

That such a claim can be made at all is remarkable, because cephalopods almost never leave any trace in the fossil record. Unlike fish, marine reptiles, or even ammonites, most cephalopods have no hard parts like bones.

Octopuses, in particular, are almost entirely “skin bags” filled with water. When they die, they rot quickly, and even the few hard parts, such as the beak, are seldom preserved.

This creates a systematic bias that skews our understanding of ancient ecosystems: animals that preserve well dominate our reconstructions, and the animals that don’t, even if they were common among certain ancient ecosystems, are largely invisible to us.

Every fossil cephalopod, therefore, represents a vital piece of palaeontological information, giving us a fleeting glimpse into a lost world of squishy invertebrates.

But not all cephalopodologists are convinced by the size estimate, with the potential length of 19m in particular drawing scrutiny on social media.

Scaling cephalopod body sizes from beaks is not straightforward. The relationship between jaw dimensions and total body size varies considerably across cephalopod species, a problem compounded by the patchy data available for rarely caught deep-water cirrate octopuses.

Other researchers have also questioned the behavioural inferences drawn from the wear patterns, arguing that bite asymmetry can be caused by many factors, and that drawing conclusions about animal intelligence from a single specimen is premature.

It is also important to put this finding into context of the living relatives of Nanaimoteuthis. Modern cirrate octopuses are not known to swim after prey, typically hunting small invertebrates on the seafloor, raising questions about whether their giant ancient cousins would ever have encountered, let alone challenged, the formidable marine reptiles.

But step back from the debate over metres and scaling equations, and something fundamental comes into view. Our reconstructions of ancient ecosystems are shaped by what preserves (bones, shells, teeth) and often systematically blind to what doesn’t.

While future investigations may test the size estimate or refine behavioural interpretations, this remarkable fossil shows that there may have been giants lurking in the vast, deep, and dark waters of the ancient oceans. We just couldn’t see them until now.

Thomas Clements, Lecturer, University of Reading

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

How are keys made, and how do they open locks? – Noli, age 12, Wisconsin

Have you ever wondered how keys work? I teach a course in computer security where we learn how locks function – and also how they can be broken or bypassed. We do this because locks teach important principles about security in general.

If you look closely at a key, you’ll see its top edge has a bunch of V-shaped valleys. If you inspect the key more closely, perhaps with a ruler, you’ll notice the bottoms of these valleys are equally spaced. The depth of the valleys encodes a sequence that is accepted by the lock, with each valley contributing one value to the combination.

Inside the lock is a cylinder – the part that moves when you stick your key in and turn it. The key can turn only if all its valleys are the right depth for your particular lock.

But how does your lock detect whether your key’s valleys have the right sequence of depths?

Inside the lock are vertical shafts, one over each valley of the key. In each shaft is a pair of metal pins that can freely slide up and down. Depending on where the pins are, they can block the cylinder from turning and prevent the lock from opening. This happens whenever a pin is partially sticking into or out of the cylinder.

When you stick a key in the lock, the pins fall into the valleys. If a valley is too high, it causes a pin to stick out and jam the cylinder. If a valley is too low, the pin sinks too low and the pin above it will sink into the cylinder and jam it. However, if the right key is inserted with the valleys at just the right depths, none of the pins get in the way.

Keys are made by placing a blank key into a grinding machine that is programmed to carve out the exact valleys that are needed. A locksmith can also change a lock by removing its pins and fitting it with new ones to match a chosen key.

In computer security, we say that security relies on “something you know, something you have or something you are.” A password is an example of something you know. A key is an example of something you have. A fingerprint would be an example of something you are. But as you can see, a key is also very much like a password, except it is encoded by grinding a piece of metal.

For this reason, you shouldn’t ever post a picture of your house key on the internet. That would be like posting a picture of a credit card or a password – someone could use the photo to duplicate the key.

It is also possible to unlock or “pick” locks without a key. By sliding a thin piece of metal into the cylinder and gently pushing the pins to the correct height one by one, locks can be opened. However, it takes a great deal of skill and practice to do this.

What does this teach us about security? First, we must make keys secret by making a very large number of possible keys, so that the right one is hard to guess or build. It’s the same for passwords. Second, it’s important to engineer a lock or computer program that requires every bit of the key or password to be exactly correct.

It’s important to study the inner workings of locks and computer programs to understand how their design might allow someone to break them.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Scott Craver, Associate Professor of Electrical and Computer Engineering, Binghamton University, State University of New York

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

Where does beach sand come from? – Sly M., age 6, Cambridge, Massachusetts

There’s more to beach sand than meets the eye. It has stories to tell about the land, and an epic journey to the sea. That’s because mountains end their lives as sand on beaches.

Over time, mountains erode. The mud, sand, gravel, cobbles and boulders they shed are washed into streams, which come together to form rivers. As they flow down to the sea, all this sediment is ground up and worn down in nature’s version of a rock tumbler.

Big rocks break down into smaller pieces, so most of what reaches the sea is mud. These silt and clay particles are too small to perceive with the naked eye. But you can see individual grains of sand, which are just bigger bits of rock.

Next time you’re at the beach, pick up a handful of sand and look closely at it. Are all the grains the same color, or a rainbow assortment? Are they jagged and angular, or smooth and round?

Different colors of sand come from different minerals, like khaki feldspar, smoky white quartz, green olivine or black basalt. The mix of colors in beach sand tells you what kinds of rocks produced it.

The shape of sand grains also provides clues about where they come from. Angular grains of the same type of sand have not traveled as far as smooth round grains, which have been more worn down. And weak rocks break down to mud faster than hard rocks, so sand tends to be made of the harder types that break down slowly.

About a tenth of the supply of sediment that reaches the sea is sand. These particles are between about half a millimeter and 2 millimeters in size – roughly as thick as a penny. These particles are large enough that they don’t flow right out to the deep sea.

But the beach is just a temporary stop for sand. Big waves pull it offshore, and smaller waves push it along the coast. So keeping a beach nourished with sand is essential for keeping it sandy.

Yet today many beaches are starving. Many dams trap the sand that flows down rivers, piling it up in reservoirs. All in all, human activity has cut off about half the sand that would otherwise end up on the world’s beaches.

But humans haven’t turned the waves off, so as beach sand washes away and isn’t replenished, the shoreline erodes. That means that many beaches around the world are shrinking, slowly but surely.

So next time you dig your toes into beach sand think about the epic journey it took to arrive beneath your feet. Take a moment to think about where the sand came from and where it’s going.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

David R. Montgomery, Professor of Earth and Space Sciences, University of Washington

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

Why does gravity pull us down and not up? - Gracie, age 9, Brookline, Massachusetts

Gravity is the reason things with mass or energy are attracted to each other. It is why apples fall toward the ground and planets orbit stars.

Magnets attract some types of metals, but they can also push other magnets away. So how come you feel only the pull of gravity?

In 1915, Albert Einstein figured out the answer when he published his theory of general relativity. The reason gravity pulls you toward the ground is that all objects with mass, like our Earth, actually bend and curve the fabric of the universe, called spacetime. That curvature is what you feel as gravity.

What is spacetime?

Before getting into the complicated world of gravity, you need to understand spacetime.

Spacetime is exactly what it sounds like: the three dimensions of space – length, width and height – combined with the fourth dimension – time. Using some very brilliant math, Einstein was the first person to realize that the laws of physics work in a universe where space and time are merged together.

What this means is that space and time are connected – if you move really fast through space, time slows down for you compared to someone who is moving slowly. This is why astronauts – who are moving very fast in space – age a tiny bit more slowly than people on Earth.

Matter makes gravity wells, not gravity hills

Remember, gravity is the idea that objects in the universe are attracted to each other because spacetime is bent and curved. When Einstein came up with general relativity, he showed that all stuff in the universe can curve spacetime – in physics terms that stuff is mass and energy.

Since your brain usually thinks about the world in three dimensions, it is really hard to think about the four dimensions of spacetime as a single idea. So to make it easier to visualize, imagine the surface of a trampoline. If there is nothing on it, it is flat. But if you stand on the trampoline, it stretches around your feet and creates a valley with you at the center. If there is a ball on the trampoline, it would roll toward your feet.

This is a two-dimensional example of how spacetime works. Your mass stretched the trampoline, creating what is called a gravity well that the ball rolls into. This is very similar to how the gravity of a heavy object – like the Earth – pulls things like you and me toward it.

To make things even weirder, since space and time are connected, time is also stretched by heavy objects!

The heavier you are, the steeper the sides of the trampoline well. That is why really massive things in the universe – like the Sun or black holes – have stronger gravity than Earth.

So why does gravity pull you down and not push you away?

Imagine someone went under the trampoline and pushed up. The ball would roll away! This would be a gravity hill, not a gravity well. As far as scientists know, matter – or stuff – always makes gravity wells and not gravity hills. Scientists can imagine things made of exotic matter or energy that would cause gravity to push you off into space, but so far, no one has found anything that could cause gravity to push you away from Earth.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Mario Borunda, Associate Professor of Physics, Oklahoma State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

Why are planets round? – Daniel B., La Crosse, Wisconsin

The ancient Greeks proved over 2,000 years ago that the Earth was round and figured out how big it was by using simple observations of the Sun.

But how do people know this today? When you drop anything, gravity causes it to fall directly toward the center of the Earth, at least until it hits the ground. Gravity is a force that is caused by nearly everything that has mass. Mass is a measure of how much material there is in anything. It could be in the form of rocks, water, metal, people – anything. Everything material has mass, and therefore everything causes gravity. Gravity always pulls toward the center of mass.

The Earth and all planets are round because when the planets formed, they were composed of molten material – essentially very hot liquid. Since gravity always points toward the center of a mass, it squeezed the stuff the Earth is made of equally in all directions and formed a ball. When the Earth cooled down and became a solid, it was a round ball. If the Earth didn’t spin, then it would have been a perfectly round planet. Scientists call something that is perfectly round in all directions a “sphere.”

The gas cloud that the Earth was made from was slowly rotating in one direction around an axis. The top and bottom of this axis are the north and south poles of Earth.

Now, hold out your right hand. Point your thumb on your right hand straight up, and curl your fingers around the direction of rotation. Your thumb is pointing toward the North pole. The equator is defined as the plane, halfway between the North and South Poles.

If you ever played on a merry-go-round, you know that the spinning merry-go-round tends to throw you off. The faster it spins, the harder it is to stay on. This tendency to be flung off is called centrifugal force and pushes the mass on the equator outward. This makes the planet bulge at the equator.

The faster the spin, the more unround it becomes. Then, when it cools and hardens, it retains that shape. If a molten planet starts off spinning faster, it would be less round and have a bigger bulge.

The planet Saturn is very oblate – non-spherical – because it rotates very fast. Because of gravity, all planets are round, and because they rotate at different rates, some have fatter equators than their poles. So the shape of the planet and the speed and direction that it rotates depends on the initial condition of the material out of which it forms.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

James Webb, Professor and Director, Stocker AstroScience Center for Physics; Stocker AstroScience Center, Florida International University

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com.

Why is water wet? – Philip S., age 12, Northville, Michigan

Spring is often a rainy season. If you get caught in a downpour without an umbrella, you will quickly learn what it means to be wet. But what is it about water that makes it wet?

I am an atmospheric scientist, and water is a fundamental part of the atmosphere. I study storms and wildfires, both of which are closely connected to water.

Why water is wet has to do with how water molecules interact with each other and the things around them.

Wet you can see

Imagine you accidentally spill water on your clothes one day. You will notice two things: First, the water spreads out on the cloth, and the wet part sticks to your body more than the dry part does; and second, the wet area feels cool.

Wet clothes stick to your body and water spreads across the fabric because water molecules are strongly attracted to other molecules, a chemical property called adhesion.

One important reason why water molecules are so attracted to other molecules is that they’re polar. Like a microscopic magnet, one end of the molecule carries a small negative charge, while the other end carries a small positive charge.

Many everyday materials, such as glass, skin and clothing, are also polar. When water touches these surfaces, the electric charges on those materials attract the water molecules and hold them in place. This strong attraction also helps water spread out over surfaces. Whether something feels “wet” to you has to do with how good a liquid is at staying in contact with a surface. Water feels wet because its molecules stick tightly to each other and to your skin.

Compared to water, mercury has much weaker attraction to surfaces. Mercury’s molecules are much more attracted to each other, meaning they have very strong cohesion. As a result, mercury does not easily stick to other surfaces.

The cool feeling of being wet comes from evaporation. Liquids need energy to change into gas because they must overcome the forces holding molecules together before they can float away. They take this energy from their surroundings in the form of heat.

When you step out of a pool and the water on your swimsuit evaporates, you might feel cold because it’s taking away heat from your body. Wet things often feel cool because evaporation takes heat away from the skin. Sometimes something that feels cool can trick you into thinking it’s also wet, even if no liquid is actually present.

Evaporative cooling is very useful in daily life, and other liquids can also do it. For example, when you clean a wound with an alcohol wipe, it also feels cool. Like water, alcohol evaporates and carries heat away from your body. Similarly, when sweat evaporates, it removes heat from your body and cools you down.

Wet you cannot see

Sometimes you can feel damp even when you don’t see any water. This is related to the amount of water vapor in the air, also called humidity.

Air can hold only a limited amount of water vapor. When there is already a lot of water vapor in the air, evaporation slows down. This makes it harder for sweat on your skin to evaporate, so you feel sticky and wet.

When air becomes completely full of water vapor, the vapor starts to condense and turn back into liquid water to form dew or fog.

How much water vapor air can hold depends on temperature. Warm air can hold more water vapor, while cold air can hold less. As temperature increases, water molecules gain more energy and can more easily escape their attraction to each other and become a vapor.

This is why dark or shady places often feel damp. These areas get less sunlight, stay cooler and cannot hold much water vapor. As a result, water does not evaporate easily and the area stays wet.

A lot of water, but not wet

Because the air’s ability to hold water depends on temperature, sometimes the air can contain a lot of water vapor but you don’t feel wet.

For example, when you are near a fire, the burning process produces water vapor. However, because the temperature is also higher, the air can hold more water vapor. This speeds up evaporation. If there are wet clothes nearby, they may actually dry more quickly.

In weather forecasts, scientists use relative humidity to describe how humid the air feels, rather than the actual amount of water vapor in the air.

Because hot air can hold so much moisture that relative humidity stays low, people are often surprised when I tell them that wildfires release large amounts of water vapor. Fire is the last thing most people associate with being wet.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Yunyao Li, Assistant Professor of Earth and Environmental Sciences, University of Texas at Arlington

This article is republished from The Conversation under a Creative Commons license. Read the original article.