Growing out of your own container
Why Do Animals Shed Their Skin?
A snake crawls out of its skin in one piece. A crab walks out of its shell. Both do it because they cannot grow inside it.
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Step 1 of 5
A hard outside cannot stretch.
You cannot have both, and every armoured animal pays for it Armour is only armour if it is stiff, and a stiff case cannot get bigger. Any animal built this way runs out of room inside its own covering.
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Step 2 of 5
So a new one is grown underneath.
Before shedding, the animal builds a soft new layer inside the old one, folded and wrinkled. Only when it is finished does the old case split open.
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Step 3 of 5
Then it swells before it sets.
The moment it is out, the animal pumps itself up with air or water so the new soft case hardens at a larger size, leaving room to grow into.
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Step 4 of 5
The soft days are dangerous.
For hours or days the new case is soft and useless as armour. Most animals hide completely through this, because almost anything can eat them.
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Step 5 of 5
A snake does it in one piece.
A snake rubs its nose until the old skin splits, then crawls forward out of it, turning the whole thing inside out like pulling off a sock.
A snake's eyes are covered by a clear scale, which is shed too. Just before moulting they go cloudy and the snake can barely see.
The short version
Animals with a rigid outside must shed it to grow, and the hours before the new one hardens are the most dangerous of their lives.
Try it yourself
Look at a snail's shell — it spirals outward instead. That is the other answer to the same problem.