ELI5: Explain Like I'm 5

Ruppeiner geometry

Okay kiddo, imagine you have a bunch of tiny particles moving around inside a box. Each of these particles has an energy, which means how fast and how much they're moving around.

Now, a scientist named Ruppeiner came up with a really cool idea to describe how the energy of all these particles is related. He drew a picture of the box on a piece of paper, but instead of just showing the position of the particles, he drew these squiggly lines all over the place.

These squiggly lines are called "geodesics," which is a fancy word that means the shortest path between two points. But these geodesics aren't straight lines like you might expect. Instead, they curve and twist all around the box.

So what do these geodesics have to do with the energy of the particles? Well, it turns out that the more energy the particles have, the closer together the geodesics get. And the less energy the particles have, the more spread out the geodesics are.

This is kind of like if you have a bunch of ants crawling around on a sugar cube. If there's a lot of sugar on the cube, the ants will all be clustered together trying to get at it. But if there's only a little bit of sugar left, the ants will be spread out all over the cube, not as interested in finding the small amount of sugar.

So, in summary, Ruppeiner geometry is a way of using squiggly lines called geodesics to show how the energy of tiny particles inside a box is related. The closer together the geodesics are, the more energy the particles have. Pretty cool, huh?