It's a beautiful warm spring day and a man and his wife are at the zoo.
She's wearing a cute, loose-fitting, pink spring dress,
sleeveless with straps.
As they walk through the ape exhibit and pass in front of a very large gorilla, the gorilla goes ape.
He jumps up on the bars, holding on with one hand (and 2 feet), grunting and pounding his chest with the free hand. He is obviously excited at the pretty lady in the wavy dress. The husband, noticing the excitement, suggests that his wife tease the poor fellow.
The husband suggests she pucker her lips, wiggle her bottom, and play along.
She does and Mr. Gorilla gets even more excited, making noises that would wake the dead.
Then the husband suggests that she let one of her straps fall, she does, and Mr. Gorilla is just about to tear the bars down.
"Now try lifting your dress up your thighs" ... this drives the gorilla absolutely crazy.

Then, quickly the husband grabs his wife by the hair, rips open the door to the cage, slings her in with the gorilla and says,

"Now, tell HIM you have a headache."

and that's a very good joke to tell whenever one needs to explain why the hobby exists!  

That and every time she leaves town she takes all the pussy with her.