Measuring one particle instantly influences the state of the other, no matter how far apart.
Imagine two magic dice that, no matter how far apart they are thrown, always show linked numbers. Bell showed with a precise calculation that no hidden trick can explain this — the link is genuinely real. Einstein called it spooky action at a distance and did not like it at all.
Why it matters Quantum entanglement is now the basis for ultra-secure communication technologies.