In the classical world, if a ball hits a wall and doesn’t have enough energy, it stops. But in the quantum world, a particle can sometimes “tunnel” through a barrier it normally couldn’t pass. This is called quantum tunneling.

It’s as if the particle suddenly appears on the other side of the wall—even without enough energy to climb over it. This happens because quantum particles are described by wavefunctions that have a small chance of existing on the other side.

Quantum tunneling is used in real technology, like scanning tunneling microscopes and even inside the Sun, where it helps fuel nuclear reactions