How did you rustproof your bottom?

My bottom is fine, thank you. LOL. That reminds me of a customer who came into the shop I worked in as an assistant manager many years ago. She evidently had her son's car. She looked me right in the face and said, "Can you grease my son's rear end?" I could not help myself. I burst out laughing, and she did too. It was hilarious. She eventually said, "You know what I mean." So we put some 90 weight lube in the differential of her son's car. She became a regular customer, and every time she came in, we had a good laugh.