Why do seashells have spirals?
For grown-ups: the science
Seashells spiral because the animal inside adds bigger new shell around its opening as it grows.
Many marine mollusks, such as snails, build shells by secreting calcium carbonate at the shell opening. Growth is often proportional, so the expanding opening follows a curved path around earlier shell, producing a spiral shape.
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Transcript
A little shell rests on the beach, with a curly line around it. Why does it grow that way?
A snail-like animal lives inside many spiral shells. Its soft body needs a hard home for protection.
As the animal grows, it adds new shell around the opening. The newest part is bigger than the part before it.
The opening keeps growing outward and around the old shell. That makes the shell curl into a spiral.
So a spiral shell is a growing home. Each new layer gives its animal more room.