The math behind road lanes. When a lane exits from the main road, the number of remaining lanes must decrease by one. When a track is added via a ramp, the total increases by one again.
Biffa: "Oh this junction is chock-a-block! What it needs is some lane mathmatics!"
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?"