Turn Signal

A set of lights on a vehicle that, when activated by the driver, flash in pairs in the direction of a turn.

Dumbass 1: is that where we're supposed to turn?"
Dumbass 2: "Yes! Put your turn signal on!"
Dumbass 1: "why would I ever do that???"
Dumbass 2: " so people behind us know that we're going to be making a turn!!!'