This made me laugh because I was the tickler in one of these situations once.
Fresh out of high school and hanging out with my old classmates we all piled on a chair to get a picture taken (pre-digital so who knows if it even was ever developed).
I was sitting in the chair and as people piled on top they were kind of across the arms of it so I had no one actually "on" me and my arms and hands were free. (See where this is going?)
Within easy reach were two, dear , female friends who suddenly got the giggles as I found their sides and ribs to lightly tickle. All while yelling out "Who is doing that?!?!" "Stop tickling!!"
I'd have to imagine they *knew* who it was...but it was fun and funny nonetheless!
thanks for bringing back that memory!
~ toyou