It's kid's game. It start when at least 2 people say the same thing at the same time. Than say "찌찌뽕" and try to either pinch the other person first.

"찌찌뽕" is a simple and childish game beginning in without a notice while two or more people having a talk. When two people by accident speak the same word (or thing) at the same time, one of the two declares "찌찌뽕" quickly, and touch the opposite's nipples(or teats). If they are not the same sex, just speak "찌찌뽕". "찌찌" means "a nipple", you know. It is a word used for very young children or infants.