Peri: Doctor, why do you wear a stick of celery in your lapel?
The Doctor: Does it offend you?
Peri: No, just curious.
The Doctor: Safety precaution. I’m allergic to certain gases in the praxis range of the spectrum.
Peri: Well, how does the celery help?
The Doctor: If the gas is present, the celery turns purple.
Peri: And then what do you do?
The Doctor: I eat the celery. If nothing else, I’m sure it’s good for my teeth.