The Harlem Shake is a dance that is still cool. The purpose of this dance is to prove you are stupid and then post it on the internet. The first people to do it were awesome, but then lotsa other people did it and it turned stupid. But it's still cool because we think it's funny to annoy people. It's sorta like Rickrolling people. You tell them to click on a link, it's the Harlem Shake, and they hate you.
In order to complete the Harlem Shake, you must get a group of people. Then you must sit there like an idiot. When the guy says "DO THE HARLEM SHAKE", you and your people start dancing like an idiot. Basically you act like an idiot while music is playing in the background. It's a pretty sweet dance.
Unfortunately, Harlem Shake's population rates down. It's not popular anymore in YouTube.