A blonde replaced all the windows in her house.

She had expensive, double-insulated energy efficient windows put in. 

Twelve months later she gets a call from the contractor, complaining that the work has been done for a year and she has failed to pay for the work done.

The blonde replies, "Now don't try to pull a fast one on me. Just because I'm blonde doesn't mean that I'm automatically stupid. The salesman who sold me those told me that in one year they would pay for themselves