On one side of a river there are three misssionaries and three cannibals. There is a boat which can be used to transfer people from one side of the river to the other. No more than two people can fit in the boat and it must have at least one person in it during any transfer. The problem is to find the shortest sequence of transfers which gets all six people from one side to the other without ever creating a situation where missionaries outnumber cannibals on either side of the river. (The idea being that if this happens, the missionaries will then be able to corrupt the cannibals by 'converting' them.)
The usual aim is to avoid the cannibals outnumbering the missionaries and eating them. But apparently that's not politically correct.