Throw a needle through a pane of glass, and the glass will break. At least, it will if you throw it with the strength and precision of Shaolin monk Feng Fei.
Fei demonstrated his ability to throw a needle hard enough that it pierces a pane of glass, popping a balloon on the other side, in a video produced for YouTube by The Slow Mo Guys. In some of the slow-motion shots of the ultrafast trick, it appears that the needle actually sticks through the pane. In others, it looks like it just cracks the glass with enough force to send shards flying into the balloon.
In either case, it's a remarkable feat, as The Slow Mo Guys demonstrated when they attempted it themselves, and only managed to put a small nick in the glass. But how does it work? [7 Amazing Superhuman Feats]
Without knowing the exact mass, thickness and chemical makeup of the glass and needle involved, it's difficult to say exactly how fast the needle was moving or with how much force Fei threw it. But here's what we do know: Glass has some highly unusual properties that make throwing a needle through it a very different task than throwing a needle through wood or metal.
The trick is getting those glass molecules moving
James Sethna is a theoretical physicist at Cornell University, and he devotes much of his research to the strange properties and fracture of glass. He told Live Science that the trick to breaking glass is overcoming its initial resistance to breaking.
"Glass is extremely tough to break," he said, "unless it has a crack. As soon as it develops a crack it's very weak."
This is because of its strange chemical structure.
In a solid metal, the molecules aren't especially well-ordered, and easily mush around one another under pressure. Crystals such as ice or diamonds form rigid, tough-to-break molecular patterns. When glass freezes though, no patterned structure forms, but the molecules still form rigid covalent bonds with one another. They don't have the tough molecular arrangement of an organized crystal, but aren't flexible enough to deal with much shifting.
"If you have an intact window pane, and you push on it, your finger is pressing against the glass, and the bonds in the [whole pane of] glass have to hold themselves in place, even though you're just pushing on this one part," he said.
The glass immediately beneath your fingertip uses its bonds to distribute some of the force outward to its neighbors, which distribute some to their neighbors, and so on.
Once the glass does start to crack though — when that system of distribution fails and the bonds begin to snap — the results can be catastrophic.
"If you have a crack in [a pane of glass you're pressing on], then the load can't get across the crack. So it has to go sideways along the crack until it gets to the crack tip, and at the crack tip then all of a sudden all of that load gets focused into a little region," Sethna said.
On the level of chemical bonds, he said, glass is actually an incredibly strong substance, less susceptible to microscopic cracks than steel. But it doesn't mush. So once just a few of its bonds snap, it's much easier for the rest of the structure to give way.
(Incidentally, this isn't just true of silica glass, which makes up most windows and is what most people think of when they think of glass. Glass includes all such rigid, chaotically organized solids — including, Sethna mentioned, hard candy.)
In order to poke through glass, Sethna said, the needle would have to be very rigid itself, and not bend under pressure.
"If somebody throws a rubber ball at your window, even very, very hard, it's not likely to break," he said.
But a rock of the same weight, covered in edges and points, thrown just as hard, will distribute its force to a narrower point of contact and not bend when it strikes. And it's likely to shatter the glass.
The trick, Sethna said, to putting a needle through glass is this: A firm needle, thrown hard enough to meaningfully crack the glass. Once a deep crack has been made, it won't take much force at all to carry it the rest of the way through.
Originally published on Live Science.