all 3 of mine use the cat flap to see who coming and going but I have only ever had one broken by them and that was years ago by Bracken - I came down one morning thinking it was rather cold in the kitchen, went to the back door to let him out and noticed that there was a hole with no cat flap, looked all around and couldn't see where it was?!?!? Bracken went out, did his business, came back in, I put his breakfast down for him only to see the missing cat flap stuck firmly around his neck

what a job I had getting it off, poor boy nearly had his ears torn off in the process
