"They arrange themselves in an orderly queue, the biggest at the front, the smallest at the back; they're lining up with one aim: to exchange properties. But none of the crabs can make a move because the chain is not yet complete. They're all waiting for the right-sized crab to come along."

The Hermit Crab vacancy chain, as featured in the 'Home' episode of Life Story, on the BBC.

[Video Link via Reddit]