Before when I was learning Biology, I got interested by an experiment in a book. It was about snails and how thing in the outside world can affect its behavior.

we lived in an apartment building back then but luckily we got a yard below us so we caught two snails easily. We put the snails inside a plastic container and after a few days, we discovered three more little snails! we started the experiment by touching it with a stick – it went back into its shell every time. we did a few more experiments and then one day ALL of the snails are gone! there are no shells in the container and it still remains a mystery to this day.