It all starts when one person randomly yells COCK! when they are in a group of people. Someone else in the group needs to catch up on what they are doing and yell AND! and a third person needs to finish off the game by yelling BALLS! It must be totally spontaneous.
PENIS-AND-TESTICLES may also be shouted, but COCK-AND-BALLS is a classic.
Scream is as loud as you can!