Q: Why did the lady wear a helmet every time she ate?
A: She was on a crash diet!
Explanation: A crash diet is a diet where a person loses a lot of weight in a short amount of time: lose weight fast. A helmet is protection for your head when you are playing a sport or working.
This joke is funny because it combines the idea of crashing (bumping into something really hard) and being on a diet. You don’t need a helmet when you are on a diet and that is why this joke is funny.