The Coolidge Effect is a phenomenon whereby males exhibit high sexual performance given the introduction of new willing females.
It earned its name many years ago when President Coolidge and his wife were touring a farm. While the President was elsewhere, the farmer proudly showed Mrs. Coolidge a rooster that "could copulate with hens all day long, day after day." Mrs. Coolidge coyly suggested that the farmer tell that to Mr. Coolidge, which he did.
The President thought for a moment and then inquired, "With the same hen?"
"No, sir," replied the farmer.
"Tell that to Mrs. Coolidge," retorted the President.
Zing! (via defective yeti)