system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the scissor glide when you cut a paper and why does it glide only after you’ve cut a little bit?
gpt
**TL;DR:** *A loosely held paper edge has some thickness, so hits scissor blades with too many paper fibres at once and causes more of a crushing than cutting action; tight paper and scissors that are already traveling create lots-of-scissor-blade-edge-to-little-bit-of-fibre contact and replace that with clean, slicing force instead.*

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Take a really close look at paper and you'll see that it's made up of closely interwoven fibres. Better paper has more and longer fibres, but most of it is a mesh of randomly stuck-together incredibly thin pieces of, well, wood. And it has a little thickness to it, not much, but a little.

So if you grab a pair of reasonably sharp scissors and put them at the edge of a piece of loosely held paper, there's not a very clean contact at that edge where the fibres of the paper move along the scissors' shearing edge to the point where they get cut. It's kind of like cutting steak by pressing your knife down on it instead of sawing; the meat gets squashed a little rather than severed because too many fibres in the meat are interacting with too little edge of your knife. Same with the paper because even though it's very thin, it still has SOME thickness to it. So it's hard to get your scissors and gliding motion started, and you end up with a crinkled little ripped area on your paper.

Two ways to fix this are to hold the paper taut, so there's a very clean contact between both the top and bottom fibres at the paper's edge and the two cutting surfaces of the scissors, or to make a small cut and have the scissors build up some velocity and then carry that through into a glide. Either way causes the fibres at the top and bottom of the paper sheet to travel ALONG the cutting blade surfaces of the scissors while preventing too many fibres from jamming against the cutting surfaces all at once.

Now you are getting LOTS of cutting surface handling a LITTLE bit of paper fibres at once, so they can slice rather than crush, and that's much much better for cutting. And then you get that satisfying *whi-i-i-i-ish* of the scissors travelling effortlessly in a perfect straight line, and the crowd goes wild!

P.S. this is also why scissors with a little damage to the blades really suck at glide-cutting. Even the tiniest of dents in the cutting surface becomes a place where lots of fibres can clog up, and suddenly you're ripping things worse than a competitive event at an all-you-can-eat chili buffet.