onions
Explanation
The Joke
A scientist proudly announces "Behold! I've genetically modified an onion so that it doesn't make people cry any more!" A person then cuts the onion and confirms "It works!" with a big smile and dry eyes. In the final panel, the person turns to the scientist and says "Science is so amazing," at which point the scientist angrily shouts "Stop that!"
The joke hinges on an ambiguity: the scientist created a non-tear-inducing onion, but the test subject's reaction -- expressing genuine wonder and emotional appreciation for science -- is itself a kind of emotional response that the scientist apparently finds just as unwelcome. The scientist wanted to eliminate the crying, but the effusive praise ("Science is so amazing") is treated as equally annoying or inappropriate, prompting the frustrated "Stop that!"
The Humor
The humor works on the contrast between the scientist's pride in their creation and their immediate irritation when someone responds with heartfelt awe. It plays on the stereotype of the emotionally detached scientist who is comfortable with technical achievement but deeply uncomfortable with sincere emotional reactions. There is also an ironic layer: the scientist successfully prevented one emotional response (tears from onions) only to be confronted with another emotional response (wonder and admiration) that they find equally intolerable.