The War Game Dice Fake-Out
My dad has these old war simulation games that involve little bits of cardboard printed with various bits of info, placed and moved around on printed paper maps of various landmasses overlaid with hexagonal grids.
One day, my dad was in the mood to play one of these, so he set up the game map on a board on some tray tables, and asked me for a die. But instead of a real die, I gave him one of a set of trick dice that I bought a long time ago. One is printed with all fives, the other with twos and sixes. (Roll a seven or eleven every time!)
He rolled the die once... "Five." Then he made a move. He rolled the die again... "Five." Then he made a move. He rolled the die again... "Five. ...Wait a minute!"
We laughed, and I went and got him a real die.
The Locker Switcheroo
The high school I went to was HSPVA, the High School for the Performing and Visual Arts. I was in the Visual part of that. We had these big lockers made of white formica; easy to clean and capable of holding lots of art supplies and huge boards and pads.
I had an upper locker; the locker below mine was shared by Richard Becerra and Demian Fore. I happened to notice that they used the same brand of combination lock as I did. And one day, by standing nonchalantly a few feet away and surreptitiously watching Demian open his locker, I was able to learn his combination.
The rest writes itself. I switched 'em! Next time Demian went to his locker, he tried to open it... but it wouldn't open. He tried it again... it still wouldn't open! A third time... still nope! Then he went to get Richard to try it.
Um... at that point I chickened out and switched 'em back, thus ending the prank. Richard came up and opened it with no trouble, merely leaving Demian mildly confused. Still, it was funny while it lasted.