Knudsen number

Okay, so imagine you are playing with a toy car on a really long and straight road. The road is so long that there is no air or wind around, it's just you and the car. Now, if you try to push the car really hard, it will start moving faster and faster, right?

But what if, instead of just pushing it, you add some really tiny, invisible balls around it, kind of like a gas? This gas is so tiny and small that you can't even see it, but it's there. When you push the car now, it won't move as fast as before, because this gas will start slowing it down.

The Knudsen number is like a way to measure how much this gas will slow down the car or any other object moving through it. Basically, it depends on two things: how big the tiny gas molecules are, and how much space there is between each molecule (kind of like the gaps between small rocks on a beach).

So, if the molecules are really small and there is a lot of space between them, the Knudsen number will be bigger, which means the object moving through this gas will be slowed down more. But if the molecules are bigger and there is less space between them, the Knudsen number will be smaller, so the object won't be slowed down as much.

In summary, the Knudsen number is a way to measure how much a tiny gas will slow down an object moving through it, based on the size of its molecules and how spaced out they are.