Although their shells are waterproof, snails' bodies are likely to dry out fast when there is a drought. Most snails are active at night or in wet weather. At other times, they rest in places where water hangs around for a long time. In very dry conditions, many snails make a door for the entrance to their shell. This door, called an epiphragm, is made of a thick layer of dried mucus. A snail can survive for months inside this protection, until damp conditions return.