Two levers and a travelling point
Why Do Scissors Cut When One Blade Cannot?
Scissors are a pair of levers sharing one pivot, and they cut at a single moving point rather than along a whole edge.
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Step 1 of 5
Each blade is a lever.
The screw is the pivot. Your fingers push a long way round on the handle side, and the blade moves a short way with a much bigger force.
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Step 2 of 5
One blade alone would only push.
A single edge pressed against paper just carries it along. It needs something on the other side pushing back before the material has anywhere to fail.
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Step 3 of 5
They only ever touch at one point.
The blades are very slightly bent towards each other, so contact happens at a single spot rather than along the whole length. All the force lands there.
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Step 4 of 5
That point runs outwards as they close.
The cut is not made all at once. The contact point slides from the pivot towards the tips, cutting a little at a time, which is why a long snip feels smooth.
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Step 5 of 5
Near the pivot is where the strength is.
The closer to the screw, the shorter the blade lever and the greater the force. That is why you push thick card right into the crook and not out at the tips.
Left-handed scissors are not just mirrored handles — the blades are swapped too, because a left hand's natural squeeze would otherwise push the blades apart.
The short version
Scissors cut because two levers squeeze from opposite sides at a single point of contact that travels from the pivot to the tips.
Try it yourself
Close a pair of scissors slowly and watch where the blades touch. The meeting point runs steadily away from the screw.
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