Two mothers met in a street.  They started talking about their children.  "I have three sons," said the first mother.  "How old are they?"  asked the second.

"Well, it's easy to figure this out," replied the first mother.  "The product of their ages is equal to 36 and the sum of their ages is equal to the number of windows in the house across the street." 

The second mother thought for a while and said, "What you've told me is not sufficient to solve your problem."    "Oh, yes, the oldest one is red-haired," replied the first mother and the second solved the problem.