Okay, here's kind of a gross one, but for guys this should be funny. I was in basic training at Lackland AFB in 1976. The last week of basic we were given town passes to enjoy ourselves (it was a Saturday). So a few of us wound up near several X-rated theaters, some of which had discounts for G.I.'s. Since we were a bunch of 18 year olds and never saw a porn before, we decided to go for it.
I think there were 6 or 7 of us, 3 sat in one row and we other 3 or 4 sat behind them. Keep in mind we still had our short, short hair. The guy sitting in front of me was kind of the geek of the group, so we wanted to have some fun. As the action on the screen started, I nudged the guy next to me and said "watch", then proceeded to tap my finger on my forearm, which kind of sounded as if I was beating off. I noticed the geek in front perked up a bit, probably wondering what that sound was. Pretty soon the money shot was on the screen, so I put some spit on my fingers and slapped it on the back of his neck. The geek just shot up out of his seat and did his WTF routine while we just sat there cracking up. It was a sight to see.
We were asked to leave after that...