A hermit crab relies on its shell to protect the soft end-portion of its body from harm. In the wild, other scavenger animals like gulls and even feral cats may try to eat a hermit crab. In cases like this, it's the shell that keeps the hermit crab from being devoured by a hungry predator. A hermit crab is not attached to its shell. The crab continues to grow throughout its life by molting. Therefore the crab must seek out a new shell whenever it outgrows its old one. That's where you, the hermit crab owner, come in. You must provide your hermit crab with several extra shells to change into, when it is inclined to do so. Your best bet is to get shells at a shell store or gift shop. You can also find shells on the beach. But no matter where you get them, you should disinfect them. Boil them in water for 5-10 minutes and let them dry before putting them in your crab's tank. This kills all bacteria and insects that may be in the shell. If you're lucky, you may witness your hermit crab changing shells. First he will inspect the shell he chooses. The crab may stick his claws inside the shell, and move the shell around a bit. If he verifies that the shell will fit, he'll try it on. He quickly exits his old shell and zips into the new one. The crab makes the switch so fast, in fact, that if you blink you may miss it. A hermit crab is very cautious like that. He knows that when he comes out of his shell, he's vulnerable to being harmed by a predatory animal or other peril. If the crab likes his new shell, he'll go about his business. If not, he'll quickly switch back and either look for another shell or decide he doesn't want a new one yet.