Second grade teacher here. Though we don't get into the weeds in our science class for obvious reasons, we've talked about the three forms of matter: liquid, solid, gas. Now, that may in fact be scientifically ridiculous to a 21st century physicist, but it's what our eyes behold, so it's what we teach little children. Students taking physics classes, if they're mandatory at the high school level (not sure), would no doubt learn about quantum theory. However, if you don't take a class like that, if you don't read about physics on your own, if you generally just don't care but were pressed to say what "stuff" is made of, you'd probably here some version of "atoms and things like that." With "things" being a solid particle, akin to a cell or even a grain of sand, constituting everything else.