Three missionaries (m1, m2, m3) and three cannibals (c1, c2, c3) want to cross a river.  There is a boat that can hold only two people at a time, and any combination of missionaries and cannibals can row the boat.  However, there is a catch.  If the cannibals on either side of the river outnumber the missionaries, the cannibals will eat the missionaries, and the game will end.  To win the game, a player must find a schedule of crossings that gets all missionaries and cannibals across the river by step 12.