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Suppose you drop a ball down the side of a valley - classical wisdom tells us that when
the ball rolls up the hill on the other side, it can't go any higher than the height from
which you dropped it - that's conservation of energy!
Even if there's a nice big long slope to roll down on the far side of the mountain, the
ball just can't get there - unless you give it enough energy to get over the barrier.
But in quantum mechanics, things work a little differently.
You see, the quantum world is probabilistic, so if you release a particle in a valley,
chances are the next time you see it, it'll still be somewhere in that valley.
But if there's a nice big slope to roll down on the far side of the mountain… well, that's
a place the particle would really like to be - and it turns out there's also a small
chance that's where you'll find it!
If this isn't crazy enough, it's even possible you'll find the particle in the middle of
the mountain… and in real life this means that an electron sometimes hangs around inside
the nucleus of an atom!