A bit messy but a real laugh.
Get enough eggs so there is one per player. Hard boil them all except one. About 15 minutes in boiling water will be plenty. Put all the eggs in a bowl, including the single egg you did not hard boil.
Now start the game. Everyone picks an egg. You then have the choice of 'switching eggs' where you and someone else switch.
Finally everyone smashes the egg against their forehead - with the inevitable laughter as one person gets a surprise!
Contributed by Yaba Yaba
Click this animated image!