Albert Einstein was famously sceptical about certain predictions of Quantum Mechanics, such as the idea that there is a special type of correlation, called "entanglement", through which acting on a particle here can influence another particle far away. It is now well established that entanglement exists, and the infamous “spooky action at a distance” that goes with it has been observed in many experiments. It’s not easy to explain what entanglement is, but it’s possible to explain what it is not: it’s not a form of classical correlation. This can be illustrated with a simple example – a piece of paper and four numbers on it!