gradient
Explanation
This comic takes the scientific concept of a chemical gradient and places it in an everyday social setting.
A person with long hair and glasses walks into a coffee shop and announces, "Good morning. I have followed your chemical gradient in the direction of increasing intensity." In biology, organisms like bacteria use chemotaxis to navigate toward food sources by following chemical gradients -- moving in the direction where the concentration of a chemical (like a nutrient) increases. This character is describing smelling coffee and walking toward the source using the language of cellular biology.
The caption -- "Biologists are no longer welcome in my coffee shop" -- delivers the punchline. The barista or shop owner is so put off by this overly technical and slightly creepy way of saying "I smelled your coffee and came in" that they've banned biologists entirely.
The humor comes from the contrast between the mundane act of following the smell of coffee and the clinical, scientific language used to describe it. It also plays on the stereotype of scientists being socially oblivious, unable to distinguish between appropriate technical jargon and normal human conversation.