Quantum entanglement is when two particles become correlated so that measuring one instantly determines the state of the other — no matter how far apart they are.
**Simple analogy:** Imagine two magic gloves, one placed in New York and one in Tokyo. When you open the New York box and find a left glove, you instantly know the Tokyo glove is a right glove — even without looking. Entanglement is similar, but stranger: the gloves don't "decide" which hand they are until one is observed.
**Key points:** - Entangled particles don't "send signals" to each other — this doesn't allow faster-than-light communication - Measuring one particle collapses both into definite states simultaneously - Einstein called it "spooky action at a distance" — he didn't like it but it's been experimentally confirmed many times
It's the basis for quantum computing and quantum cryptography.