Why does a snail have a shell?
For grown-ups: the science
It keeps its soft body from drying out and protects it from sun and wind.
A snail's shell is an exoskeleton made mostly of calcium carbonate, formed by its mantle as part of its body. Besides protecting against predators and physical injury, pulling into the shell also helps reduce water loss. It isn't perfectly sealed, though, so resistance to drying out varies by species and environment. Even newly hatched snails already have a tiny shell, which grows larger as they do.
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Transcript
A snail always carries its shell on its back. What's that shell for, anyway?
A snail's body is soft and full of moisture. Its shell shields it from sun and wind, so it doesn't dry out.
When it gets too hot or something scary comes near, the snail tucks inside its shell. It curls up small and rests safely there.
The shell is connected to the snail's body and grows bigger along with it. Even a tiny baby snail already has its own little shell.
A snail's shell is like a little house that protects its soft body. That's why it always carries it on its back.