Patient to doctor: "Will I be ok, Doc?"
Doctor: "I doubt it. Mercury is in Uranus now."
Patient: "I don't do that astrology stuff."
Doctor: "Nor me. My thermometer just broke."
Mercury in Uranus
Explanation
The humor is based on the double meaning of "Uranus". The doctor uses it to make the patient believe it is an astrological reference, while in reality, the doctor is talking about the planet Uranus, which is where his thermometer ended up.
wren
i love that joke