What is it with people?

[Untouch]

Happens quite a lot with girls / young women, they generally are messing about or got dared to do it.
One of the times it happened to me was 2 years ago, I went to a university open day and was going to get lunch and a girl slapped my ass, I turned around and the girl smilied then the group of girls she was with laughed.
But after I got lunch I was going back and the group of girls where still there so I slapped the one on the ass who got me, it was quite funny as she never expected it.
Next year I actually joined that university and in the first week I was in the student's union and that girl came over and asked if I was that guy. I never remembered her but she remembered me. :p We're now good friends and go to the gym each week together.