Peri and The Doctor in “The Caves of Androzani”

Peri

Doctor, why do you wear a stick of celery in your lapel?

Does it offend you?

Peri

No, just curious.

Safety precaution. I'm allergic to certain gases in the praxis range of the spectrum.

Peri

Well, how does the celery help?

If the gas is present, the celery turns purple.

Peri

And then what do you do?

I eat the celery. If nothing else, I'm sure it's good for my teeth.