This is from “Jokes Central” [jokes777@hotmail.com]. It was posted on the groaners listserv.

A professor of a Midwestern medical school loves to tell tall tales.

He once regaled his students with the case of a young boy who grew so fast that his shadow couldn’t keep pace with him. That boy grew and grew until he had to get on a chair to button his collar. Some months later, when his head had grown through the top of his hat, his doctor said, “Son, you’ll have to eat lard.”

“But why lard?” the kid asked.

“It’s time for shortening,” was the reply.