My favorite riddles:
Q. What’s brown and sticky?
A stick!
Q. What’s a foot long and slippery?
A slipper!
Q. What do you call a boomerang that doesn’t come back when you throw it?
A stick!
Daniel
Thanks, CalMeacham. According to this site:
*Homer was a 9th century Greek poet. Remember Homer’s Odyssey? After the Trojan war a king had a long journey home facing monsters, magic, and enchantresses. Well, legend has it that Homer killed himself because he could not answer a simple riddle spoken by Greek fishermen below.
• What we caught we threw away; what we didn’t catch, we kept.*
It was the fishermen part that led me to think it was fishing nets. But I figured the idiom didn’t date back that long. 
Apparently ‘lice’ is also an acceptable answer. 
But I guess you shouldn’t just replace any parasitic noun in there.
Ahhhh. Must be A wound. Not that anyone would cut their own…
If you want riddles that people won’t really find amusing (because they don’t tell them like this anymore), you could always try the Exeter Book Riddles:
http://www.technozen.com/exeter/
On the plus side, some of them are risque, in a “Turtles” kind of way. (They sound dirty, but have “innocent” answers). On the other hand, too many of them (at least in the Penguin translation I have) have as their answer “Creation”. I imagine this Medieval king threatening to cut off his court jester’s body parts if the answer to the riddle turns out to be 'Creation" one more time.
some of the risque riddles:
A fart.
A good followup:
What’s invisible and smells like a carrot?
rabbit farts
Daniel
A dumb riddle I made up and that an eleven-year-old should appreciate:
Q: Why is Lincoln’s head on the penny?Because if it was his feet you wouldn’t know who it is.