ELI5: Explain Like I'm 5

Hilbert's first problem

Okay, so imagine you're playing with building blocks. You can stack them up to build towers, right? Well, famous mathematician David Hilbert had a problem with building towers, but it's not as simple as just stacking blocks.

Instead, imagine building towers made up of lines in space. These lines can't cross or intersect - they need to be straight and never change direction. Hilbert wondered, is it possible to build towers of any height using only these lines without them crossing or intersecting?

This is what we call Hilbert's First Problem. Mathematicians worked on this problem for a long time, but in 1900, Hilbert even offered a prize of 1000 marks (a lot of money back then!) to anyone who could solve it.

Eventually, a mathematician named Max Dehn showed that it was impossible to build these towers infinitely tall without lines crossing or intersecting. However, we can still build towers of a certain height using these lines without any problem.

So, Hilbert's First Problem was all about figuring out if we could build towers of any height using straight lines without them crossing or intersecting. Even though it turned out to be impossible, it was still an important problem for mathematicians to work on and helped advance our understanding of geometry.
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