Quantum tunnelling
Quantum tunnelling is a part of the theoretical branch of physics known as quantum mechanics. It states that  electrons can behave like both particles and waves, and can cancel the effects of an energy barrier if the energy barrier is thin enough, due to quantum mechanics being  dependent on probability. In other words, particles can travel through walls, doors, etc. if the door or wall is thin enough. The particle—usually a subatomic or quark particle such as an electron or photon—can become a wave to focus a great deal of energy on the barrier, ultimately negating it. There is also a non-zero chance that it could be found inside the barrier. However, this only noticeably works for subatomic particles, as the chance for macroscopic objects to do this is very, very low, small enough to not be thought about much.