Okay, here's my favourite:

Two mathematicians are sitting in a restaurant. One says "I've got three daughters, and if you multiply their ages together you get 36."
The second one says: "Give me another clue."
First one "If you add their ages together, you get the same number as the number of tables in this restaurant."
Second one: "Give me another clue."
First one: "The eldest one is blonde."
Now the second mathematician replies with the correct ages. What are the ages of the three daughters?

There's no trick to this, just some logical deduction is required.