"Ribbed" joke

Billy Joe goes to the drugstore for some condoms.

He goes up to the druggist and asks, "Sir, can you tell me where the ribbed condoms are?"

The druggist replied, "Son, do you know what condoms are used for?"

"Sure do," replied Billy Joe. "They keep you from getting venereal diseases."

"OK," said the druggist. "Do you know what the ribs are for?"

Billy Joe thought for a minute, then looked up at the druggist and replied, "Well, not exactly. But they sure do make the hair on my goat's back stand up."