This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
Three humans, one big monkey and two small monkeys are to cross a river:
a) Only humans and the big monkey can row the boat.
b) At all times, the number of human on either side of the
river must be GREATER OR EQUAL to the number of monkeys
on THAT side. ( Or else the humans will be eaten by the monkeys!)
The three columns represent the left bank, the boat, and the right bank
respectively. The < or > indicates the direction of motion of the boat.
HHHMmm . . HHHm Mm> . HHHm <M m HHH Mm> m HHH <M mm HM HH> mm HM <Hm Hm Hm HM> Hm Hm <Hm HM mm HH> HM mm <M HHH m Mm> HHH m <M HHHm . Mm> HHHm . . HHHMmm