Why do earthworms come out when it rains?
For grown-ups: the science
When rain fills the air pockets in the soil, worms crawl up to the ground where they can breathe more easily.
Earthworms have no lungs; they absorb oxygen through their moist skin and release carbon dioxide the same way. When heavy rain fills the soil's air pockets with water, oxygen is much scarcer in water than in air, making it harder for worms to exchange gases underground, so many move to the surface. Wet ground also keeps their skin from drying out, which helps them travel. Not every worm surfaces for the exact same reason - vibrations from rain and easier movement for spreading out may also play a part.
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Transcript
On a rainy day, you might spot a worm on the sidewalk. Why do they come out of the ground like that?
Earthworms live in tiny little gaps down in the dirt. Their damp skin soaks up air right there underground.
When lots of rain falls, those tiny gaps fill up with water. With less air around, it gets hard for worms to breathe.
So the worms crawl up onto the ground where there's air. The wet ground keeps their skin nice and damp too.
Worms come out to find a spot where breathing is easier.