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?!