You know you don't have enough energy to throw it through the wall, so you always expect it to bounce back. With something as large as a rubber ball, though, that probability is so small that you could throw the ball for billions of years and never see it go through the wall. But with something as tiny as an electron, tunneling is an everyday occurrence. On the flip side of tunneling, when a particle encounters a drop in energy there is a small probability that it will be reflected. The Heisenberg uncertainty principle People are familiar with measuring things in the macroscopic world around them.
Someone pulls out a tape measure and determines the length of a table.
A state trooper aims his radar gun at a car and knows what direction the car is traveling, as well as how fast. They get the information they want and don't worry whether the measurement itself has changed what they were measuring. At the atomic scale of quantum mechanics, however, measurement becomes a very delicate process. Werner Heisenberg was the first to realize that certain pairs of measurements have an intrinsic uncertainty associated with them. For instance, if you have a very good idea of where something is located, then, to a certain degree, you must have a poor idea of how fast it is moving or in what direction.