Having younger kids, I field a lot of questions. My impression is that the subject matter, when it touches on school subjects, is weighted toward science. Maybe that's my impression because science is not my strong suit, and it nettles me when I can't with confidence answer, and then I remember being nettled. On the topic of my impressions, it also seems that curiosity declines as people age. We get used to the way everything is and, even if something registers dully on our consciousness, we don't wonder about it.
The above is of course introductory to an incident from life. Yesterday I was asked why, when the weather is cold, can you see exhaust coming out of the tailpipes of cars, but you can't when the temperature is "normal." I thought I had at least a vague idea about this, but when I started to speak I smacked into a wall made of the fact that nouns and verbs tend to be specific, and when uneasiness compels you to take off the hard edge of their meaning with modifiers you are well on your way to producing an unsatisfactory speech act.
But now we have The Internet! Water is a byproduct of the combustion process. Usually it's so hot that it's emitted invisibly in its gaseous state. But on a cold day at the end of the tailpipe it condenses into something visible—what you can see is tiny droplets as the water acquires its liquid form. It's sort of hard to talk about this because most of us use terms like "vapor" and "steam" interchangeably to refer to "visible foggy mist of water too hot to be liquid." But think of a teapot of boiling water on the stove. We can see what we usually call steam coming out of the spout. If however you look closely the foggy mist is only visible a little ways out from the spout: right at the point of emission, it's too hot to be seen. But it would be different if it was really cold out there!
Anyway, that is the state of my understanding after reading here and here. Comment if you know better. Possibly I will someday be asked about water, on a cold day, trickling out of the tailpipe of the car in front of you as it pulls away from a formerly red light. Most likely such a car isn't far from where it started out and its exhaust system is still so cold that the water got all the way to liquid form, making a little pool in the bottom of the tailpipe before running out when the vehicle moves forward again.
But why does the forward motion make it run out the rear?
It's a notion simultaneously "romantic" and plausible that the best scientists are able to maintain into intellectual maturity a childlike sense of wonder. To most of us oldsters, kids are themselves like everything else in the world, their strangeness and private thought life invisible until very occasionally it shines forth to memorable effect. A friend of mine told me that the first time his young son was going to ride on an airplane, he got really quiet at the airport, and broke his silence only when, walking through that tunnel between the gate and the plane's door, he asked: "Is this where they will shrink us?"
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