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Put your hand flat on a table and press. Your hand stops. But the atoms in your hand and the atoms in the table never get anywhere near each other — and they never will.
Your hand is on the table. Press harder. It still stops.
Whatever is holding your hand up is doing a real job — you can lean your whole weight on it and it does not give. It feels like the simplest thing in the world. Two hard objects met, and neither would move.
That is not what happened. Nothing met.
When most people imagine a solid, they picture something packed absolutely full — tiny balls jammed shoulder to shoulder with no room left. Press on that and of course you stop. There is nowhere to go.
A real solid is not like that at all. A solid is mostly nothing.
And there is a second problem, which is the more interesting one. Even if you did shrink down and go looking for the place where your hand meets the table, you would not find a surface. Atoms do not have skins. There is no outside edge to bump into.
So before anything gets explained, it is worth going down there and having a look.
Start at the part of your fingertip actually resting on the table, and go down. Each press is roughly a hundredfold jump in magnification. Watch for the row where you expect a surface and there isn't one.
You press your finger on a table and it stops dead. Given what the ladder showed, what is actually stopping it?
The outermost thing an atom has is its electrons, and every electron carries a negative electric charge.
Negative charges push each other away. Always. You have felt this with magnets — two like poles refusing to meet, sliding off sideways, getting harder to close the gap the nearer you bring them. Electric charge does the same, and it is the nearer part that matters here.
As the gap between your atoms and the table's atoms shrinks, that push does not grow gently. It grows viciously. Halve the distance and you get far more than double the push. Which means there is a distance where the push exactly matches however hard you are pressing — and that is where your finger stops.
Press twice as hard and the gap narrows a little, and the push climbs to match you again. That is what hard means. Not that something is full. That the pushing back rises faster than you can push.
You are not feeling stuff. You are feeling a force — electrons refusing to be near other electrons, getting more stubborn the closer they get.
This is worth doing rather than reading. Take two magnets and turn them so they refuse each other, then push them together slowly.
There is a distance where they stop. Push harder and the gap shrinks a bit and then holds again. Let go and they spring apart. At no point do the magnets meet, and at no point does the resistance feel vague or soft — it feels like a wall, and it is a wall, made of nothing but push.
That is your hand on the table, scaled up until you can watch it. The only difference is the size of the gap. Between the magnets it is centimetres, so you can see the emptiness. Between your hand and the table it is about one atom across, so you cannot.
And because you cannot see it, your brain reports the whole thing as contact.
Try this fact on a physicist and there is a fair chance she will shrug and say: of course you touch the table. That push IS touching. What did you think touching was?
She is not disagreeing with a single thing in this episode. She agrees the atoms stay apart. She agrees about the gap, the shove and the refusal.
What she is saying is that the word touch never meant "atoms make contact" in the first place — nobody who invented that word had ever heard of an atom. It meant: I pressed on something and it pushed back. And that is happening. So by the only definition anyone has ever actually used, you touched it.
This is worth noticing, because it is a completely different kind of disagreement from the one earlier in this episode. "A solid is packed full" is a claim about the world, and it is false. "You do / don't really touch things" is an argument about what a word covers, and both sides predict exactly the same experiment. Telling those two apart is a genuinely useful skill, and most arguments people have are secretly the second kind.
Every statement below is a true fact about atoms. Only some of them explain why the table pushed back on your hand. Sort them and read what each one says.
Tap an item, then tap where it belongs
A physicist tells you: "You obviously touch the table. That push-back is what touching has always meant." Is she contradicting this episode?
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Scale of the atom: a typical atom is on the order of 0.1-0.3 nanometres (about a ten-millionth of a millimetre) across, while a nucleus is on the order of a few femtometres — roughly 1/100,000 of the atom's width, which is the basis of the standard marble-in-a-stadium comparison. Nearly all of an atom's mass is in the nucleus: protons and neutrons are each about 1,800 times the mass of an electron. Human skin cells (corneocytes of the outer epidermis) are on the order of tens of micrometres across — roughly 0.03 mm — which is why they are visible only under a microscope. Contact forces between everyday objects are electromagnetic: the negatively charged electron clouds of the two surfaces repel one another, and the repulsion rises very steeply as separation decreases (standard treatment of normal/contact forces in introductory physics). Note for parents and older readers: electric repulsion is not the complete account of why solids resist being compressed — quantum mechanics contributes as well. This episode deliberately stops at the electromagnetic explanation, which is correct as far as it goes, and leaves the fuller treatment to a later age band. That everyday 'touching' is a name for this repulsion, rather than a claim about atomic contact, is a point made by many physicists; it is a dispute about the meaning of the word, not about any measurable prediction.