When you cut something,
what do the atoms do?

The full narration of the video.

You press the knife into a tomato. The skin holds, and the tomato just squashes. Switch it up, draw the blade back and forth, and it sinks right in. Same knife. So what changed? The answer is under the edge. Let's follow the blade and zoom all the way in.

At a thousand times, a sharp edge isn't a line at all. It's a rounded rim, about a hundred and fiftieth the width of a hair. Zoom further and you can count atoms. Even a good kitchen knife is about two thousand atoms wide at the edge. The thinnest razor blade is still around a hundred. The edge is far bigger than an atom: it can't split atoms in half.

So what does it actually cut? Look at the tomato side. Inside the tomato, atoms hold on to each other, like kids on a playground standing in line, holding hands. Cutting never chops the kids. It pulls their hands apart.

The edge is so narrow that all your push lands on one thin line. Press with the weight of a bag of sugar, and at first contact the pressure under the edge can top a thousand times air pressure. The hands stretch tighter and tighter. The first to give are the ones right in front of the edge. Snap. They let go. The gap opens before the blade even touches them. All the knife does is keep prying that gap wider and pushing it ahead.

Then why does pressing straight down squash it? The tomato is soft. Push, and the whole thing gives way. The force spreads out, and the hands stretch without breaking. Draw the blade, and the edge scrapes sideways along the skin. The patch right underneath gets yanked hardest, so a gap opens fast, before the tomato can squash. In 2012, researchers in France and the US experimented with soft jelly: same idea. Though that was jelly, not tomatoes.

Push the two halves back together. Will they stick? No. The moment a fresh cut meets air, its free hands get grabbed by stuff in the air, like slipping on gloves. Fresh-cut metal: covered in under a hundred-millionth of a second. This is where the playground idea breaks down: kids can just hold hands again, but an atom's free hand gets taken by something else.

And with no air? In space, two clean pieces of metal that touch can stick for good: welded without heat. It's called cold welding. Last, the knife itself. Its edge atoms get pushed and pulled too. After enough cuts, the edge curls and chips, and the knife goes dull. So next time you slice a tomato, don't bear down. Draw the knife back and forth.

Then come back and tell us: when you pressed straight down, did your tomato squash first, or split first?