I realize maybe I got a little ahead of myself -- I should start simple, and actually, this is a question with an answer that bugs me.
My first day of freshman chemistry, chemistry was defined as "The study of change". It didn't make much sense to me at the time. I thought chemistry was about elements, beakers, drugs, and explosives. Not that I started studying chemistry for these reasons, that's just what came to mind. So, when defining chemistry, I expected "The study of the elements" or "The study of chemicals" -- but "The study of change"? Not at all.
Now I think I'm beginning to glimpse the reason for this definition. In the first semester, we dealt with chemical reactions like sodium hydroxide with water, which dissolves and heats the water -- a temperature change. We saw liquids combine to form solids -- a phase change. We observed as pink indicator disappeared in a beaker, indicating how acidic our solution was -- a color change. In essence, everything in chemistry can be related back to directly observing changes.
Still, isn't that what all science is about? A block changes position, but its physics. A group of flies change genes, but that's biology. We come up with explanations for change all the time -- so why does chemistry get labeled "The Study of Change"?
It bugs me. I don't think that the description really elucidates what students are about to study, and even after a fair amount of core material, I sit mildly perplexed. So why give a definition like this? I think I'd much prefer something along the lines of "Chemistry is the study of atoms". It's frank, direct, and while a bit boring, honest. While chemistry no longer claims to look at the fundamental pieces of the universe, that does not really matter. We study atoms. We study how they interact with one another, and while that is not the fundamental makings of our world, everything is still made of them, and understanding how they interact is understanding our universe. Just in a different way than, say, the general theory of relativity.
Still, boring definitions are not a great way to start off a class. You want to inspire wonder, awe, and excitement for the things to come, as well as engage minds to start thinking about the subject matter. Saying "Chemistry is the study of atoms" doesn't exactly teach very many people anything new. So, I offer an alternate definition:
Chemistry is the study of how a massive number of minuscule particles interact on the atomic level and what effect that has in our everyday world.
Maybe a bit cloggy, but still better than "We're gonna talk about atoms, which originated from the mind of Democritus, but Aristotle blahblahblah..." Alright, I'm hamming it up, but still: There has to be something that actually describes what we're going to talk about in terms that people can understand, while simultaneously not bringing it down to a level where people don't grow as students. That's the great challenge of teaching: We must become experts in a field, understand it, and then be able to to inspire how we constructed these ideas from the bottom up in a group of individuals with differing minds and perspectives. So, while I won't say my definition is the best, I will say it's gotta be better than "Change" and "Atoms".