Turtles come in all shapes and sizes, yet they all have one thing uniquely in common … a shell. So why do turtles carry their home with them wherever they go? Are they hiding pizza in there, or is that just what the cartoon version of turtles makes us believe? For answers, we turn to the wonderful people at The Smithsonian.

The benefit of a turtle’s shell is clear. It’s armor that moves with turtles and protects them from animals that want to make them lunch. But how did it get there in the first place?

To understand how they managed to evolve their hard exterior, scientists and paleontologists had first determined which animal “family” ancient turtles came from: snakes, birds, lizards or even dinosaurs. Many people now agree that turtles came primarily from the lizard and snake families around the time that all land on Earth was one big continent known as Pangea. This is why we find turtles in nearly every corner of the world today.

Early versions of a turtle’s shell were large, hard ribs that not only protected the ancestor of the turtle, the Pappochelys. These bones also helped with buoyancy—the ability to move up and down in water. If you’ve ever seen a turtle in water, you can see how they move much better than on land, a benefit of having a buoyant shell with them.

Over millions of years, the rib bones began to get harder and larger. This is important because, unlike other creatures with hard exteriors, like armadillos, a turtle shell is actually part of the bone structure of the animal, not scales. It’s almost like they’re wearing their bones on the outside of their body which is why you can’t take a turtle out of its shell—the shell is part of the turtle.

Turtles have shells because they found a really good way to protect their insides. They started out as big rib bones and grew from there. The big, heavy armor they carry around makes them slow, but you don’t need to be fast when you basically have a tank surrounding you!