Black holes are perhaps the most perfectly thermal objects in the universe. Identifying the physical mechanism that explains their entropy is a long-standing problem in theoretical physics. In this talk I discuss a recent result on entanglement and thermalization that resolves this puzzle and can have far-reaching implications. The mechanism was first found in the context of loop quantum gravity, but it survives in the perturbative regime. I consider the entanglement entropy of gravitons and matter across the horizon of a black hole. Remarkably, in low-energy physical processes, the change in the entanglement entropy reproduces the Bekenstein-Hawking area law: causal horizons are hot and have an entropy because of entanglement. The universality of the result relies on a quantum version of Einstein's equivalence principle.