ELI5: Explain Like I'm 5

Hopf invariant

Okay kiddo, let me explain what Hopf invariant means. So, imagine you have a balloon. If you stretch the rubber of the balloon and tie a knot in it, you create something called a "hedgehog" or "hopf point". It basically looks like a bunch of spikes sticking out of the balloon in all different directions.

Now, the Hopf Invariant is a way of measuring how many spikes the hedgehog has. You know how you count your fingers to see how many you have? Well, the Hopf Invariant counts the number of spikes on the hedgehog.

But, it's not actually counting the spikes! Instead, it uses some fancy math to figure out how many times the spikes wind around each other. You see, the spikes on the hedgehog can go over or under each other, and if you follow one spike all the way around to where it started, you might find that it's gone around another spike a certain number of times.

The Hopf Invariant tells us how many times this happens for all the spikes on the hedgehog. It's a bit like counting how many times a rope goes around a pole - but with lots of ropes and lots of poles!

Scientists use the Hopf Invariant to study things like knots and shapes in higher dimensions. It might sound complicated, but mathematicians think it's really cool!