At some point in GCSE Chemistry class, the teacher will probably say that all atoms are made of three basic particles: protons, neutrons and electrons. She's right to say so, by the way. The same way a mars bar is made of caramel, chocolate and that wierd foamy stuff. But obviously you could go one level deeper and ask what the chocolate, caramel and foam are themselves made of. And so it goes with atoms.
Yes protons, neutrons and electrons are the atomic ingredients and they're all you need to explain most of Chemistry, but the rabbit hole of particle physics goes a lot deeper my friends.
Protons and Neutrons, the scourge of every GCSE student's life, are made up of smaller particles called quarks (usually pronounced kwark in the UK, but the guy who named them pronounces it kwork, so we should probably do likewise). Quarks come in six different varieties and their combinations are what make up the more familiar protons and neutrons.
But what about the quarks? I hear you say. And what about the electrons? Well the answer is that we're pretty confident these particles are "fundamental" which means they don't have any internal structure. In other words, they are the smallest things.
This starts to broil the brain after a while because to say something is the smallest thing means you couldn't chop it up. If you somehow had a quark in front of you and a quark-knife, you would be unable to slice it in two. Either the quark would resist the chop, or the act of slicing it would cause it to disappear.
There's a whole list of these "fundamental particles" called The Standard Model (I did a song about it once) which describes the properties of these building blocks, but what causes them to be that way we haven't the faintest or foggiest.
Quarks and electrons could be made of smaller stuff, and in fact there are several theories which suggest they might be. The most famous of which is probably M-theory (a version of Superstring theory...which is itself a version of String theory) which argues that fundamental particles can be thought of as tiny vibrating strings made of.......???stuff?? In truth, even asking the question "what is a string made of?" might be a meaningless question, like asking how to melt water, strings are the stuff which make, they are not themselves made.
For the time being though we don't know why quarks are fundamental, what would happen if you tried to split one or why it stops there rather than going on forever. But we've built a hefty machine to help us explore this mystery. The largest man-made machine ever in fact. You might have heard of it.