Asked by: Sally Muskell, Littlehampton
Not because they might drown. People often claim that this is the explanation, yet earthworms breathe by exchanging oxygen and carbon dioxide through their skin, which they keep wet with a slimy mucus. This means they are happiest in damp soil and most species can survive for several days under water. So a brief rainstorm should not bother them. Indeed, they may even exploit the wet weather to travel longer distances than they would normally be able to underground.
Some may need to come to the surface to mate when it is wet enough, but only a very few of the 4,400 known species of earthworms do this. Another possibility is that earthworms confuse the sound of rain with a predator, such as a mole, and so make their way upwards to the surface to escape. Certainly the animals are well known to respond to such sounds, as any worm catcher will tell you. Some old tricks include vibrating sticks, saws on wooden stakes. And, more bizarrely, giving worms tea and beer.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.