Calvin and Hobbes by Bill Watterson for March 24, 1987
Calvin: This transmogrifier will turn you into anything at all. All you do is set this indicator, and the machine automatically restructures your chemical configuration. You can be an eel, a babboon, a gaint bug, or a dinosaur. Hobbes: What if you want to be something else? Calvin: I left some room. Just write it on the side.