When a shark bites a hagfish, the hagfish answers with slime: a small squirt from glands along its sides that meets the seawater and swells, in under a second, into a cloud of threads and mucus that clogs the shark’s gills. The shark lets go. Then the hagfish has a problem of its own, because it is wrapped in the same stuff and breathes the same water. So it ties itself in a knot, an ordinary overhand knot made of its own body, and slides the loop from head to tail, scraping itself clean. Most defences come with a second skill nobody mentions: getting out of them afterwards.