ELI5: Explain Like I'm 5

Banach–Tarski paradox

Hi there! So, imagine you have a piece of playdough. Can you imagine it?

Now, what if I told you that if we cut and rearranged the playdough just right, we could make two playdough pieces that are exactly the same size as the first one, but have different shapes?

Yes, that sounds pretty crazy, right? But that's kind of like what the Banach-Tarski paradox is all about, except instead of playdough we're talking about mathematical objects called spheres.

Okay, so mathematically speaking, a sphere is just a three-dimensional ball. And the Banach-Tarski paradox says that if you take one sphere and break it up into a bunch of pieces, you can rearrange those pieces to create two new spheres that are the exact same size as the original sphere!

But how does that work? Well, the key is that the pieces you break the sphere up into aren't just ordinary pieces - they're special mathematical shapes called "paradoxical decompositions". These shapes have some really weird properties, like the fact that they can be split up into smaller versions of themselves over and over again, without ever getting smaller than a certain size.

So when you break up the sphere into paradoxical pieces, you can use those pieces to create two new spheres by some really complicated math magic. It does work, but it's not something that you would ever actually do in real life because it's just a strange mathematical concept that has no real practical application.

So, the Banach-Tarski paradox might seem confusing and unrealistic, but it's actually a really interesting example of how crazy and magical math can be sometimes.