"Butter" joke

A man is showering up in a locker room with his buddy when he notices his friend has a huge penis.
"Damn Bob, you're hung!" Jim exclaims.
"I wasn't always this impressive, I had to work for it."
"What do you mean?" Jim asked.
"Well, every day for the past two years I've spent an hour each night rubbing it with butter. I know it sounds crazy but it actually made it grow 4 inches! You should try it."
Jim agrees and the two say good bye.
A few weeks later the two are in the same locker room and Bob asks Jim how his situation was.
Jim replied, "I did what you said but my penis has actually gotten smaller! I lost two inches already!"
"Did you do everything I told you? An hour each day with butter?"
"Well, butter is expensive, so I've been using Crisco."
"Crisco?" Bob exclaimed, "No wonder, man, Crisco's shortening!"