A scientist tells a pharmacist, “Give me some prepared tablets of acetylsalicylic acid.”

A scientist tells a pharmacist, “Give me some prepared tablets of acetylsalicylic acid.”
“Do you mean aspirin?” asks the pharmacist.
The scientist slaps his forehead. “That’s it!” he says. “I can never remember the name.”

This joke is an occupational joke. The critical element of the joke is that scientist is apparently able to remember a complex name for something but cannot remember the simpler name for the thing. Scientists are often portrayed as being detached from reality, so this joke plays on that.

The phenomenon of not being able to remember a normal word is fairly common, so the listener probably feels for the scientist in some way, while also recognizing that to forget the name of aspirin is pretty absurd.