system
Given the following post, respond as if you're commenting on Reddit:
human
Why are earthworms more prone to going on the sidewalk during and after rain?

I have noticed they also like to go on the sidewalk in the morning, too.  Why is that?
gpt
Apparently there are two theories on this, but the both theories are based upon the fact that earthworms do not have lungs to breathe, but rather they breathe through their skin, in a process called diffusion. For diffusion to occur, the earthworm’s skin must be kept moist, so they release mucus to keep their skin moist. **Damp** or **moist** soil also helps this process as it prevents them from drying out.

So the first theory is that they surface so that they don't drown! After/during a rainfall the ground becomes saturated with water (far beyond the point of it being moist).  Too much water doesn't help the process of diffusion as the air in the soil is replaced with water.

The second theory is that the earthworms use these periods as an optimal time for "migration". The higher moisture content in the air provides an opportunity to move greater distances across the soil surface than they could do through soil. It is less work to move on the surface than burrowing through the soil.

This second theory appears to be gaining more ground and also helps explain why earthworms make their appearance at night when the moisture in the air is higher.