Why won't a key with round bumps fit a lock with square gaps?

Have you ever held a key in your hand? It feels cool and smooth, with little bumps along one side. You push it into a door, and it slides in with a soft click.

Now picture a different key, with round bumps along its edge. Picture a lock whose gaps inside are all square. Why won't that key fit that lock?

Inside this lock sits a row of little gaps. Each gap is square, with flat sides and sharp corners, like a tiny box. The gaps are there to hold the bumps of the right key.

Now look at a key from the side. Its edge makes a line of hills and dips. That line is its outline, which means the shape all along its edge.

A key fits when its outline matches the gaps, bump for gap. Each bump slides into its own gap. It works the way a puzzle piece slides into its spot.

Our key has round bumps, curved like the top of a pebble. A round bump bulges out in the middle. The square gap waits with its flat, straight sides. So what do you think happens when the round bump pushes into the square gap?

The round bump slips into the square gap, but it can't fill it. A round bump has no corners, so the four corners of the gap stay empty. The bump wobbles loose instead of sitting snug.

The sides of the gap are made of hard metal. Hard metal won't bend or squish to hug the round bump. So the gap keeps its square shape around it.

A key fits only when every bump sits snug in its gap. Our key's outline has round bumps, and the gaps are square. So this key doesn't fit this lock.

Outline. The key's outline has round bumps that can't fill square gaps, so it won't fit.

Next time you hear a key click in a door, you can smile and know its shape fit just right, so sleep tight and dream of perfect matches.