Cat flap

The house I live in comes with a cat flap and I have no cat. I keep meaning to glue it shut as a neighbour's cat keeps popping in and although sometimes it's nice to have a cuddle and a chat I resent him using the place like a cat hotel. I just heard him at the 'door' and my brain has hit on a deterrent. I'm going to stick a shallow pan of water underneath the cat flap. Return of the evil genius. Ha, ha, ha. I'll do this when I have time to wait in for the little bugger. I want to hear him squeal.