I was going to get a brain transplant, but I changed my mind.
Why this joke works
Changing the Mind in Question
“Changed my mind” usually means reversing a decision. Because the decision concerns a brain transplant, the phrase can also suggest replacing the speaker's mind itself. The joke works best as a quick, matter-of-fact one-liner. Pause briefly after the transplant plan, then let the familiar explanation end the thought before listeners notice that it can describe the procedure too. The mind is associated with the brain, so changing the planned surgery also sounds like physically replacing the mind.
A closer look
The Decision Changes the Organ Under Discussion
“Changed my mind” is an ordinary way to say someone reversed a decision. Here, the abandoned plan is a brain transplant, so the phrase also suggests physically exchanging the very mind that made the choice. The sentence works because it stays calm and grammatical under both readings. No medical detail is needed; the familiar expression reaches the end before the surgical interpretation catches up.
Fact behind the joke
Brain Transplants Are Not an Established Human Treatment
The National Institute of Neurological Disorders and Stroke describes the brain as the control center for movement, sensation, thought, emotion, and vital functions. Modern transplantation can replace several organs, but medicine does not transplant an intact human brain. The joke treats “brain transplant” as a recognizable fictional procedure so “changed my mind” can operate both figuratively and literally.