These tiny things, like electrons or photons, have a unique quality called entanglement. When two particles become entangled, they become deeply connected. It's like they've become best friends and they care so much about each other that if anything happens to one of them, instantaneously the other one knows about it and reacts accordingly. This happens even if they are very far apart.

Bell's theorem, proposed by physicist John Bell in 1964, was built around this idea of entanglement. Bell questioned how particles could know what's happening to their partner instantly, when nothing else in the universe can move faster than the speed of light, including information.

Bell came up with a sort of test. He said that if quantum mechanics was right about entanglement, then certain types of measurements should reveal connection patterns that couldn't be explained by any theory that didn't involve some kind of super-fast, or "non-local," spooky interaction.

He developed the Bell inequalities, a mathematical tool to distinguish the predictions of quantum mechanics from those of so-called "local realistic" theories. Local realistic theories are ones where things only directly affect their immediate surroundings and information doesn't travel faster than light.

And guess what? When the tests were done, the results lined up with quantum mechanics and against the local realistic theories. This doesn't prove that information really does travel faster than light, but it does show that something weird is happening that we don't completely understand.

In conclusion, Bell's theorem highlights the mysterious, non-local nature of the quantum world. It tells us that either the world is profoundly strange on a microscopic level, or we're missing a big piece of the picture that could make it all make sense. It's a deep question that scientists are still wrestling with and it plays an essential role in our understanding of the nature of reality.