Here's a nice one:

A census taker knocks on a man's door and asks if he has children.
"I have three children" the man says.
The census taker then asks: "How old are they?"
The man, being a keen mathematician replies: "The product of their ages is 72, and the sum of their ages is the same as my door number"
"Don't be ridiculous" says the census taker, "I can't figure out their ages from that"
"Oh right" replies the man "I forgot to mention that my eldest daughter likes chocolate"

The census taker then writes down the ages of the tree children.

How old are the three children, and why?