Hey You What If? - Episode Guide

Experience the most extraordinary ride in the universe – falling into a black hole.

The deepest hole humankind ever dug only got one thousandth of the way through the earth. But what would happen if you kept digging?

What would happen if there really was a lift that took you all the way up to space?

Find out why turning into a huge giant would be no fairy tale.

Experience the ultimate in long distance travel - a holiday on Mars!

Good news - it's possible to levitate! Bad news – there are some serious side effects.

Clouds might look like cotton wool, but if they really were, things would get very messy.

Find out why stopping the earth from spinning isn't such a good idea.

Things don't quite go to plan when the moon turns into cheese.

Find out why a flat Earth would feel very unflat indeed!

The planet is being destroyed - but this episode demonstrates the things you can do to help save it.

It really is possible to time travel into the future, and the team show you how!

If you have always wanted to shrink yourself, this is the episode for you!

Some day people will live on the moon – find out just what they are in for.

What if you could turn things to gold? This episode shows that it might not work out that well.

Want to reach for the stars? It would be the journey of a lifetime - literally.

Find out why living in slow motion would let you see things no-one else can!

Would it be possible to live on an Earth shaped like a doughnut? You might be surprised!

A fun run goes wrong when one ambitious runner tries running at the speed of sound.

What happens if you try to catch a falling star? Spoiler alert - probably best not to try!

Why waking up with lizard powers would make you useless on earth but a superhero in space!

It's the ultimate baking challenge: making a gingerbread house you could actually live in.

Is it really possible to teleport? Spoiler alert: it could end in disaster!

Empty the oceans and you'd see hidden wonders, but you'd also cause the end of the world.

See what would happen if you really could switch off the sun.

Anyone can fly like a bird – you just have to travel across the solar system to do it!

Could you build a slide to the other side of the world? You would need to start in space, use the slippiest slide ever and make sure you have enough supplies before you go down it!

Wouldn't it be cool to be able to walk through walls? From turning into a gas to quantum trickery, we discover why it wouldn't be quite as useful as you'd like.

Bugs might be annoying, but we would soon regret it if we got rid of them - all our favourite foods would disappear, and there would be dung everywhere!

From cloning to 3D printing, what if we could actually make copies of ourselves?

Imagine if we had a thermostat that could turn the world temperature down - could we fix global warming? But be careful, a few degrees too far and could cause an ice age.

Imagine if we had a thermostat that could turn the world temperature down - could we fix global warming? But be careful, a few degrees too far and could cause an ice age.

Venus has acid rain, crushing pressure and is hotter than an oven, so it might not sound like the ideal place to live. However, up in the clouds it's almost just like home.

Find out how to shout from one side of the world to the other.

We try out three potential ways to become invisible and discover some surprising side effects.

There is a way that Earth could have a second sun, so we try it out to see what happens.

We look at whether some animals can get gigantic and find out that if you are being chased by a giant snail or a humungous butterfly, then fear not, you must be dreaming.

We try using a gravity-bending black hole and spinning the earth faster to see if we can make it possible to walk on the ceiling..

What would the planet be like if all other humans disappeared, leaving you as the last person on earth?

Is it possible to build a sand castle the size of a real castle? We imagine how it could be done, using a pneumatic drill and a huge, solid gold steamroller!

We find out what would happen if you shrank the earth. You could jump higher and move faster, but you would end up with altitude sickness even at sea level.

If you had a tree that could grow money, would that make you rich? Even if you wait 20 years for your tree to grow, you will probably end up no richer than you started.

Flying like a superhero may sound like enormous fun, but there are some serious downsides that Hollywood does not tell you about - weather, extreme cold and even fizzy blood.

Imagine removing all the colours one by one? It would change more than just how things look, and after taking out just three colours, we would not be able to see anything at all!

If everyone in the world ate nothing but cheeseburgers, we would run out of land, and even if we went meat-free, we would have to invent some crazy burgers to get the nutrients we need.

To throw a ball around the world, you need to start out in space, throw it ten times the speed of a bullet and make sure your ball does not explode when it hits the atmosphere.

So you want to build the tallest building on Earth? Ambitious but good on yer! This episode will tell you how, starting with how not to. Most tall buildings are tall and thin because land is expensive. But this isn't the best design because the lowest floor has to take the weight of all the floors above it. And so eventually, you reach a limit where the structure at the bottom isn't strong enough. If you want to make a building significantly higher, then you need to change the shape and instead take inspiration from wedding cakes. The bottom layers of a wedding cake get wider and wider, which means that they can take more and more weight. In fact, the tallest building ever conceived, the X-Seed 4000, is 4km high and can house a million people - but it's never been built. However, even if you made a giant cone-shaped building, and assuming you have materials that are strong enough, then eventually you would reach another limit, when the weight of the building is too much for the ground below. That's why mountains have a maximum size. So, if you want to go higher still, you need to go to the moon, where gravity is lower and so you can go six times higher. But then that wouldn't be the tallest building on Earth, would it?!

Running a marathon is hard. At least it is for a human. But what if we could swap our legs with an animal - would that make it any easier? First, we try on a pair of kangaroo legs. Rather than jump from one leg to the other, kangaroos keep their legs together and store the energy from each bounce in special ligaments that act like springs. They recycle the energy so that the kangaroo can travel long distances with very little effort. This efficient style of locomotion helps them get around the Australian bush, where it can be a long way between places with food and water. However, they get tired after around 30km and so a 42km marathon would be too much for them. So what about the longer legs of a T. rex? They have a stride length of around 4 metres compared to 1 metre for a human. And they can travel up to 50km. However, scientists believe that T. rex was too heavy to be able to run - if they tried they'd probably break their legs. So maybe we're stuck with our human legs. But don't worry - when it comes to running marathons, humans are the best animals around because we have something that other animals don't - sweat! Sweat helps keep us cool and means we can keep going for long distances without overheating. Go humans!

On the face of it, humans are a bit breakable. We have these hard skeletons that can crack if you have a tumble. But if we were indestructible, like a superhero, then we'd never have to worry about ending up in plaster! So what's the best way? First we try removing the problem - it's our bones that break, so let's get rid of them altogether. Unfortunately, this doesn't work very well. Without our bones we'd basically be blobs and unable to move anywhere. Okay, metal is tough right? How about taking inspiration from knights in armour, and making our skin solid metal? The problem with metal is that if you hit it hard enough it will actually bend. So, your metal skin would soon be covered with dents and you'd end up completely bent out of shape. What we need is something that can take the knocks without changing shape permanently. What we need is something like rubber, something 'elastic.' Elastic materials deform when you hit or stretch them but then they spring back to their original shape. In fact, there are new man-made elastic materials that can stretch up to a 100 times longer! If we were made of elastic, we'd be very hard to break but there would be a down-side. We'd be very, very bouncy and if we weren't careful we'd end up crashing around the house like an oversized rubber ball and breaking everything around us. Of course, we could always make our house elastic too?

If the beach is too far to go but you still want to spend some time surfing waves, then why not surf on land instead? Well, there are some very good reasons people don't surf on land. To surf, you need waves which are steep enough that you can literally slide down them - that's how surfing works. You slide down the face whilst the wave continuously picks you up so you can just keep sliding. Finding waves the right shape on land is quite a challenge. There are naturally occurring waves on land, but they are created as a result of dramatic seismic events like an earthquake or a volcanic eruption. And even if you were standing by a volcano when it went off, the waves it produces aren't very steep and they're not very tall. So you really couldn't surf them. Which leaves you with two alternatives: A. Melt the ground so it becomes a liquid. But then the ground would be over 600 degrees centigrade and destroy your surf board. Or B. Fluidise the ground. Fluidising involves blowing air through the soil particles so that they can move freely, which causes the ground to behave more like water. But you still need that volcano - a big lava bomb is just the ticket for setting off a big wave in the fluidised ground! Dangerous sport, land surfing...

We try out a crazy idea from science fiction to build a giant sphere around the sun and give us more energy than we'll ever need.

We take inspiration from the natural world as we imagine a new kind of superhero that can walk up walls.