Here in Japan we have an ancient tradition called kanchō. Basically, we shove our fingers up each other's arse. For a laugh, of course.

Anyone is game: Your coworker. Your boss. The cute waitress. A random office lady on the train. Everyone appreciates and partakes in this tradition!

It's easy: Just sneak up on them from behind, demonstratively clamp your hands together with both index fingers extended in the shape of a gun, and then ram your digits up their unsuspecting anus as hard as you possibly can.
Important: Always make sure to yell kanchō while performing this prank! That way it's just a fun tradition (as opposed to sexual harassment, which it would be if you forgot to say the magic word).

Then laugh it off together, as your victim will surely also find great hilarity in your application of Japanese traditions.