ELI5: Explain Like I'm 5

Löwenheim–Skolem theorem

The Löwenheim-Skolem Theorem is like a magic trick for mathematicians. It helps them understand something called first-order logic, which is like the rules for talking about things in math.

Imagine you have a big room full of toys, but you want to play with only a few of them. You can pick out your favorite toys and put them in a smaller box to play with. The Löwenheim-Skolem Theorem works kind of like that.

It says that no matter how big your toy room is, you can always find a smaller room that has the same rules for playing with toys. So, even if you don't know every toy in the big room, you can still create a small room that's just as fun to play in.

In math terms, that means if you have a really big math system (like all the numbers in the world), you can still create a smaller system (like just the even numbers) that has the same rules and still works just as well.

So, just like picking out your favorite toys to play with, mathematicians can use the Löwenheim-Skolem Theorem to pick out a smaller set of rules to work with and still get the same results.