Four or five players stand in a row, all facing forward. The first player (A) starts laughing loudly. After a few seconds, A turns to look at B while continuing to laugh. B then starts laughing too.
A turns away — and instantly stops laughing, becoming completely serious. B continues laughing, turns to C, who starts laughing. B stops. And so on down the line.
The contrast between hysterical laughter and sudden seriousness is what makes this exercise hilarious to watch and surprisingly challenging to do.