interparticle spaces

Interparticle spaces are the gaps left between a substance's particles — and, perhaps surprisingly, they contain absolutely nothing at all, not even air.

• These spaces exist in every state of matter, but their size differs hugely: barely-there in solids, a little larger in liquids, and enormous in gases, which is exactly why gases are so easy to compress and solids almost impossible to.
• They're also the reason mixing two substances doesn't always add up the way you'd expect — a smaller particle can slot into the empty spaces between the particles of a bigger one, so the combined volume can end up less than the sum of the two.
• This is different from constituent particles themselves — interparticle spaces are the empty gaps between them, not the particles.

It's a strange but useful idea: solid, "full-looking" matter is mostly empty space at the particle level, held together only by attraction, not by particles physically touching all around.