Quantum entanglement

Entanglement is a phenomenon in quantum mechanics by which two seemingly different systems can be inextricably linked. It is at the heart of the EPR paradox and the source of the phrase "spooky action at a distance" that is often attributed to Einstein, and is also the mechanism that allows quantum teleportation to be a reality. Entanglement is purely quantum mechanical; no true analog exists in the classical world. Despite this it is not too hard to invent fictional classical systems that behave as though entangled, for pedagogical purposes.

One of the most common analogies is to imagine a pair of coins that are normal in every way except that after you have flipped either one you will know with 100% certainty what the other will land on when you decide to flip it, regardless of how long you wait to flip the second or how far apart the coins are when flipped.