One of the riddles we used to have painted on the walls of one of our classrooms in secondary school (with the last part added in :+ ):
What runs but doesn't walk,
has a mouth yet still can't talk,
has a head but doesn't think,
has a bed but will not sleep
and twists yet is not a riddle?
Took us 12 year old students a couple of days to figure out back then (despite it being terribly obvious in hindsight of course). So just wondering how many minutes it will take here :P .