quantum tunneling
Quantum tunnelling is the passage of a particle through a barrier that, by the rules of classical physics, it does not have enough energy to cross. Imagine a ball rolling toward a hill too high to climb; classically it always rolls back. Quantum mechanically, there is a real chance the ball appears on the other side without ever going over the top — it tunnels straight through.
The explanation lies in the wave nature of matter. A particle is described by a wavefunction, a spread-out probability amplitude, not a hard pellet at a single point. When this wave meets a barrier it does not abruptly halt; it decays exponentially inside the barrier rather than instantly vanishing. If the barrier is thin enough, the wave is still nonzero at the far edge, which means a nonzero probability of detecting the particle beyond. Nothing is borrowed and energy stays conserved; the particle simply has a chance of being found across.
Tunnelling is not an exotic edge case but a workhorse of nature and technology. It is how the Sun shines, since protons fuse only by tunnelling through their mutual repulsion; how some radioactive nuclei decay; and how enzymes shuttle electrons in living cells. Engineered versions power scanning tunnelling microscopes, flash memory, and tunnel diodes. The effect is real, routine, and exploited daily.
Even with too little energy, a particle has an exponentially small but real chance of passing through.
Tunnelling is sometimes described as 'borrowing energy' via the uncertainty principle, but that picture is misleading. Energy is conserved; the effect comes from the wavefunction's exponential decay, and the particle emerges with the same energy it entered.