When some math geeks celebrate their birthday by having each half of their cake cut into four pieces, with three of them each having an arc length equal to the radius of the cake.
Deduce that pi slices of pie leads to the fact that “2π radians = 360 degrees.”
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"