Back when I was a FormaciΓ³ Professional student, for those of you from the UK that means after the sixth form, we always used to do this sort of joke on some of the guys in the middle of a party. I guess this is a pretty common joke but bear with me.
We would take a couple of chairs and normally select a couple of guys to be sat down in a room with all of us girls. They were blindfolded and told not to stand up. If we could do this in a private room where the other guys couldn't watch then all the better.
Then we would do our best to "provoke" them as they sat there. This meant whispering in their ear, kissing their neck, caressing their legs and so on.
The idea was for us to have fun getting them all worked up then suddenly we would pull the chairs out from under them and they would have to stand up with their hard cocks tenting their trousers. We would then all have a good laugh at them.
We did this one summer at a friend's chalet. Everyone was in swimming shorts and bikinis. We chose a quite good looking guy to have fun with (some of the girls preferred doing it to the ugly single guys but that was just cruel) it was to be only him as it was his birthday.
We were in the house in the living room whilst the rest of the guys were outside round the pool I guess there were about twelve of us with him. We took him away and placed a chair in the middle of the room. We put on some raunchy music and started making suggestive remarks.
He sat there, knowing all about the game I am sure, and so we told him the rules, that if he raised his hands the game was over. It was always a chance for some of the less popular girls to feel up some of the good looking guys and they sure weren't shy once we had got started.
One girl was running her hands through his hair as another started tonguing his ear. Others were caressing his chest and arms. Then Lisa took his left hand and placed it on her own leg. She ran his hand up the inside of her thigh really high up. I would guess his fingers brushed the crotch of her bikini bottoms at the top.