EPR paradox

The EPR Paradox is a thought experiment about a pair of particles that have a special link between them called "quantum entanglement". This means that if one particle changes, the other one will change in the same way, even if they are very far apart. The EPR Paradox asks what would happen if you measured the properties of one of the particles. According to one set of rules of physics (called "local realism"), you should be able to measure one particle without affecting the other particle. But according to another set of rules (called "quantum mechanics"), measuring one particle will affect the other particle - even if the particles are far apart. The paradox is why both sets of rules can't be true at the same time.