Tunneling

A sheep is a particle that either has enough energy to clear a wall, or it doesn't.

The atomic and subatomic entities that we call 'particles', have inherent wave properties. Waves pass through thin barriers. A rubber balloon becomes transparent (allows light waves to pass) as the balloon is inflated. In the same way, electrons may cross a thin insulating layer. They are said to tunnel through the barrier, without having enough energy to escape, in the normal sense, if they had only the properties of real particles.