ELI5: Explain Like I'm 5

3-torus

Imagine you have a toy car that can only move forward, backward, left or right but cannot go up or down. Now imagine you want the car to move in all three dimensions: forward, backward, left, right, up and down. The 3-torus is a way to solve this problem by taking a flat piece of paper and bending it around so the opposite sides connect to each other to form a cylindrical shape. Then, you take the cylinder and bend it again so the opposite ends connect to each other, creating a torus, which looks like a donut. Finally, you repeat this process one more time by taking the torus and bending it again so the opposite sides connect, forming a 3-torus.

Now, imagine you place your toy car on the surface of the 3-torus. If the car keeps moving in one direction, it will eventually loop back to its starting point after going around the entire 3-torus. This is because the 3-torus wraps around itself in such a way that there are no corners or edges, and everything loops back to its start.

The 3-torus is a mathematical concept used to describe a three-dimensional shape with a loop in it. It is often used in geometry, topology, and theoretical physics to describe complex systems, such as the behavior of particles in space. Just like how the toy car is always moving but never leaves the surface of the 3-torus, some particles in space are always moving but return to their starting point after completing a loop around the 3-torus.