when I purchased some sidewalk chalk for my little girls.
They were brighter and cheerier than your usual snail, and their slimy paths were outlined with the corresponding color.
I was mean and wouldn't let the kids use any of my cookie sheets as a lid for these slimy things. They had to think of other materials, so Kilian cut part of a cardboard box, then put a heavy phone book on top of that. The next day we found that snails can "eat" through thick cardboard, the cover and 13 pages of a phone book. Who knew?