Our dinner conversations often track across history, or science, or engineering, or all kinds of other interesting topics. I'd been telling the kids about Liberty Ships, and giving a description of the engine room I'd received from a friend who is very much into naval history.
Dad: "They had this massive one-cylinder engine for the whole ship."
This is not actually correct - it was me misremembering my friend talking about how big just one of the pistons was. The engine was also steam-powered, thus rendering a lot of the following conversation irrelevant as to Liberty Ships.
Blake, 9: "Was it two-stroke?"
Dad: "Could've been four-stroke."
Blake: "Maybe it had a flywheel."
I tried again to explain to him that they had been designed to be very simple, and that they probably wouldn't have built an engine that required a flywheel. His return implied that it must have, then, been two-stroke, and after switching from thinking about Liberty Ships to thinking about engines, I got his point.
Dad: "Oh! You're saying it couldn't be a four-stroke engine, since you'd need a second cylinder to run the non-firing part of the cycle."
Blake, patiently: "...Or, a second-cylinder analogue. Like a flywheel."
This, it turns out, is not strictly true - the Wright Brothers' first airplane had a four-stroke, one-cylinder engine (in fact, a lightweight, powerful engine was one of their key developments). I was so pleased with the phrase "a second-cylinder analogue" being casually tossed about, though, I had to write about it. In some sense, the engine itself could be considered a "flywheel analogue" in that design, in that it stores the momentum from the firing cycle and uses it to push the piston through the other three cycles.