When it's a ruler. A piece of wood can turn into a ruler and a King already is one!
Browse popular collection of riddles from links below:
brainden.com
www.doriddles.com
www.riddles.com
www.rd.com
goodriddlesnow.com
Check out ReadWriteThink.org's guide on how to write your own riddle.
Of interest, find out the 10 Hardest Logic Puzzles Ever Created, featured on ConceptisPuzzles.com.