Snails with counter-clockwise twisting shells can waste weeks trying to mate with a clockwise one.
Most snails possess a shell which is twisted clockwise.
Occasionally however, a specimen develops with its shell twisted counter-clockwise.
When a rare snail with a counter-clockwise twisted shell tries to mate with a clockwise twisted congener, both snails enter into a slimy foreplay which lasts for weeks.
But the orientation of their sexual organs makes it impossible.