Now replace the tennis ball with an electron. The electron, intuitively, would require a specific amount of energy needed to surpass a barrier, such as a gap of air. However, quantum mechanics and intuition tend not to occupy the same space! The electron, instead, always has a ''probability'' of passing through the barrier without ever having come in contact with the barrier in the first place. If this happens, the electron's probability will from there on out be lower than it was prior to tunneling through the barrier.