ELI5: Explain Like I'm 5

Monstrous moonshine

Monstrous moonshine is a strange and confusing concept, but let's try to understand it together. Have you ever played with Legos? Imagine that you have a big pile of colorful Legos of different shapes and sizes. Now, you can make different structures by putting these Legos together in different ways. Similarly, mathematicians have discovered that there are some special shapes called symmetrical or 'group' shapes that are made up of smaller building blocks called 'symmetry groups'. Just as you can change the shape of your Lego structures by rearranging the Legos, mathematicians can change one symmetry group into another by rearranging the building blocks they are made of.

Now, imagine you have two Lego piles. The first pile has a bunch of blue, green and red Legos, while the second pile has yellow Legos. What if we told you that the two piles of Legos are secretly related to each other in some way? This is similar to what mathematicians discovered with a special kind of symmetrical shape called the 'Monster group'. They found that the Monster group can be broken down into smaller symmetry groups, just like a pile of Legos can be broken down into smaller pieces. But here's the catch - they also discovered that the number of these smaller groups is exactly the same as the number of different arrangements of Legos in the challenging E8 shape! How amazing is that?

In conclusion, monstrous moonshine is a term to describe the surprising relationship between the symmetrical 'Monster group' and the E8 shape made up of Legos. It's like a big puzzle that mathematicians have solved, and it has opened up new areas of research in mathematics. It's a bit confusing, but it's also really cool!