Atoms are the fundamental building blocks of matter, which means they are very small indeed. The world at the scale of atoms and molecules is difficult to describe and hard to imagine. It is so odd that it even has its own special branch of physics, called quantum mechanics, to explain the strange things that happen there. If you were to throw a tennis ball against a brick wall, you might be surprised if the ball passed cleanly through the wall and sailed out on the other side. Yet this is the kind of thing that happens at the quantum scale. At very small scales, the properties of a material, such as colour, magnetism and the ability to conduct electricity, also change in unexpected ways.
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