Entanglement is really "just" the quantum version of this effect, which is why it‘s also called "quantum correlation". Take two entangled photons. Measuring bo

Entanglement is really "just" the quantum version of this effect, which is why it‘s also called "quantum correlation". Take two entangled photons. Measuring both polarizations with horizontal/vertical polarizers, you find that horizontal polarization (h) of the first means vertical polarization (v) of the second. So far, everything is classical and analogous to the coin example. The source could just produce the pairs h/v and v/h each 50% of the time. But now note that both h- and v-photons pass through a diagonal polarizer 50% of the time. So if you measure with diagonal polarizers you‘d classically expect no correlation. But in fact, if the first photon passes through a diagonal polarizer, the second will never pass the same diagonal polarizer, and always pass through a perpendicular-diagonal polarizer. This is impossible with just classical randomness. Bell‘s inequality describes how strong the correlations between polarizations could be with any classical model, and in reality we measure them to be stronger than that.