I have a question about the relationship between snail and rain. A few days ago I found snail shells in a moist area and I brought them in for crafts work (I thought they were empty because snail usually come out if you poke them). After a few days it rained, and I found that the snails have moved from the space i put them in (but not far). My question is why did they move after it rained outside but not when I brought them in a week before?
The movement of organisms towards water is termed hydrotaxis. 'Hydro-' refers to water and '-taxis' refers to movement. Hydrotaxis has been investigated in animals such as the nematode Caenorhabditis elegans (Wang et al., 2016) and, closer to snails, the gastropod Littorina littorea (Gendron, 1977). Animals may sense the water content of their surrounding environment and moisture content in the air to move where they are more comfortable.
Animals need specific amounts water in their bodies to allow effective metabolism. We can control our bodily water content by taking a drink but simpler animals aren't afforded that luxury so they need to move towards/away from water. Your snails likely moved because the water levels in their bodies dropped as they 'dried out' over the week. They could then have sensed a gradient in the moisture content of their surroundings and moved towards it. Mechanisms in animals' bodies can give animals responsive behaviour, as seen here. For instance, C. elegans uses its cGMP pathway, neurons and biochemistry to respond to water content with movement (Wang et al., 2016).