The first thing I drew were two stone wheels, one round and one square. 1 I didn't know where I would go with this but knew that would enable me to start associating them with different frames of reference. I quickly associated the "square" wheel with two things: a bad car or a "clunker" and a prototype of the round wheel.
Out of Place:
I combined the square and round wheels together into one 2 thereby creating the sine qua non of humor, incongruity—something out of place, odd, or surprising that sets in motion a cognitive tension that needs to be resolved. Incongruity itself is usually not enough to be funny because it needs resolution so that it makes sense in some way. There needs to be what is called an "appropriate incongruity".
I came up with the caption saying, "The back part I call 'the wheel', the front part 'the brake'." Once I did that caption, I thought it would also be incongruous if the guy who invented the wheel called it fire 3. This is actually one of the first ways children make verbal jokes. They mislabel—call a dog a cat—and so on.
In Process: I had early on associated "fire" as one of the inventions we think of when we think about the wheel, and then drew a wheel on fire and labeled it a "twofer" invention. 4 I knew that joke would need work and figured I'd come back to it later.