People say that biting off your finger would be as easy as biting a carrot if your brain didn't try to stop you. How the f do people know that and how many people's fingers did they bite off before coming to that conclusion?
Finger-Biting Experiment
Explanation
The joke plays on the absurdity of the claim that biting off a finger is easy, questioning how such a conclusion could be reached without a significant amount of people actually doing that. It combines the dark thought of self-harm with the ridiculousness of the claim, creating a morbid, funny scenario.