I was on a crowded tube when I felt somebody groping me from behind. It was too packed to be able to turn around and see who was doing it, which presumably provided him with a false sense of security. I grabbed the hand, thrust it into the air and loudly said: ‘Has anyone lost a hand? Because I found this one on my arse’. He rushed off the tube at the next stop and I think he’ll seriously think twice before doing that again.