Here's the whole idea in one breath: every bit of matter is either a pure substance or a mixture, and a pure substance is either an element or a compound. That's it — three boxes, and everything you've ever touched falls into one of them. The skill is just learning to look at the particles and see which.
Start with the Big Fork: Pure or Mixed?
Pick up anything. The first question is whether it's one substance through and through, or several substances jumbled together. A bar of pure copper is one substance everywhere you look — a pure substance. A glass of salty water is two substances sharing the same space, the salt and the water just physically mingled — a mixture. The tell is in the particles: a pure substance is made of all the same kind of particle; a mixture is a jumble of two or more different particles that aren't bonded, just keeping each other company. Because they're only neighbours, you can pull a mixture apart by physical means — filtering, evaporating, picking — with no chemistry at all.
Splitting the Pure Box: Element or Compound?
Now zoom into a pure substance and ask one more question: is every atom the same, or are there different atoms locked together? If it's all one kind of atom — copper is nothing but copper atoms, oxygen gas is nothing but oxygen atoms — it's an element, a building block that can't be split any simpler by chemistry, with its own square on the periodic table. But if different elements are chemically bonded in a fixed ratio, you've got a compound. Water is the classic one: every water particle is two hydrogens locked to one oxygen, always that exact recipe — H₂O.
The whole difference between a compound and a mixture is the bonding. In a compound the atoms are chemically joined into brand-new particles with a fixed ratio. In a mixture nothing is joined — the particles are just neighbours, and the ratio can be anything (a weak cordial and a strong cordial are the same mixture, only mixed differently).
Drag the particle pictures in the toy into the three bins. A scatter of single atoms is an element. Identical bonded pairs are still an element (same atom, just paired up). Two different atoms bonded is a compound. A jumble of two kinds, unbonded, is a mixture. Once your eye learns those four pictures, you can classify anything.