I forgot to explain why Classical/Newtonian Mechanics isn't exactly correct!

Basically: Nature is crazy when something is really small, massive, cool, fast, or any combination therein. Classical mechanics works well for most daily uses as an approximation with acceptable error, but it is not precise. Quantum mechanics, relativity, etc. fill in the gaps to provide better accuracy and a stronger understanding of the universe. They also explain effects which are classically inexplicable.

Example: You throw a tennis ball over a wall. It goes over the wall.

You throw an electron over a wall. It could very well return and hit you in the face, even though it should have made it over the wall.

You throw a tennis ball at a wall. It hits the wall and bounces back.

You throw an electron at a wall. It could very well disappear right before hitting the wall and reappear on the other side.