You won't.

"You won't" is something someone says to someone else who says they are going to try something or think they are capable of doing a specific task. For example, if a friend of yours is about to shoot a trick shot in basketball you would say "you won't." Or if some friends say they are going to jump of the pier, you say "you won't." This is not something to discourage another friend from achieve his or her goals, but simply a way to have fun. It is an inside joke that me and some of my friends created a few years ago. Warning: Do not say "You won't" to someone trying somethings obviously dangerous. If they harm themselves or others, you may feel or even be partially responsible.

Something a little different, but almost the same is the Nesbit. This is again something you can do with your friends, and precaution is needed so you don't overdose. (On fun or booze). A Nesbit is a formation of cups during a "re-rack" while playing the game of Beer or "Water" Pong.

Picture of a Nesbit:

It is a parallelogram facing you. So it is two straight lines but staggered.

THIS IS NOT MY IDEA. This is the idea of Zach Gostling.
This is a picture of a Zach Gostling:

Notice the 6' 8" quality.