People start milling around, not touching each other, just walking hither and yon, or yon and hither, whichever comes first. The instruction is: "go to the biggest empty space you can find" - which, of course, changes as soon as someone "fills" it.
Next, people are asked to select someone to be their personal "bomb." Don't tell that person. Don't even look at that person. But keep as far away as possible. This tends to increase speed and a general sense of humorous mayhem.
Then people are asked to select someone else to be their personal "shield" - moving about the area so as to keep the shield between them and their chosen bomb. This tends to make people move even faster.
Finally, people are told that the bombs will explode in ten seconds. We count backwards from ten, and mill madly in imaginary desperation.
It is to laugh.