Found posted in a humor newsgroup (rec.humor.funny).
A man called his mother in Florida. He said to his mother, "How are you doing?"
She said, "Not to good. I've been very weak." The son then asked why she was so
weak? She said it was because she hadn't eaten in 38 days. The son then asked,
"How come you havn't eaten in 38 days?" She said, "Because I didn't want my
mouth to be filled with food when you called."