Video narration
If a ball can't roll over a hill, can it appear on the other side? Classically, no. It rolls back. A particle treated as a solid ball would follow that rule. But at the smallest scales, a particle is described as a spread-out wave of possibilities-not an object with one fixed location. When that wave meets a thin barrier, it doesn't stop. It extends into it, sometimes beyond it. So there's a chance of finding the particle on the far side. That is quantum tunneling: not smashing through like a bullet or borrowing energy, but a result of wave behavior. The effect has limits. Thicker barriers and larger objects make the chance fall rapidly, so everyday walls stay solid. Yet tunneling helps explain radioactive decay, the Sun's fusion, and microscopes. So can a particle cross without enough energy to climb over? Yes-its wave spreads through the barrier.