Imagine you have a big bag of toy blocks and you want to use them to build something really amazing, like a castle or a spaceship. But you're not sure how many blocks you need or how to arrange them in the best way. That's kind of what mathematicians felt like when they were trying to solve Hilbert's twentieth problem.
Hilbert was a famous mathematician who gave a list of 23 problems that he thought were really important for other mathematicians to solve. His twentieth problem had to do with something called "group theory." This is a fancy way of talking about how different blocks (or mathematical objects) can fit together and create bigger structures.
Hilbert wanted to know if there was a way to figure out whether two different groups were really the same or not. This might not seem like a big deal, but it's actually a really hard problem.
Think about it like this: if you have two different piles of toy blocks, how do you know if they're the same or not? Maybe one pile has more blue blocks and the other has more red blocks, but they still have the same number of blocks overall. Or maybe one pile has lots of flat blocks and the other has lots of curved blocks, but they can still be rearranged to make the same thing.
Mathematicians had been trying to solve this problem for a long time, but nobody could come up with a good solution. Hilbert's twentieth problem was so important that people worked on it for more than 50 years!
Eventually, a mathematician named John Thompson figured out a way to solve it. He came up with something called Thompson's uniqueness theorem, which basically says that if two groups are really different, then there will always be some mathematical object (like a curve or a line) that can help you tell them apart.
So, using Thompson's theorem, mathematicians can now be confident that if they have two different groups, they can figure out whether they're truly different or just different-looking versions of the same thing. In other words, they can build their castles and spaceships with confidence!