When we come across something unfamiliar in the ordinary everyday world, we might prod and poke it with a stick, or smell it or taste it to see what it is. There might for instance be little resistance on our stick if the object happens to be a jellyfish, or a considerable amount of resistance if it is a lump of basalt, and this resistance tells us something about the composition of the object. This is what we do with electrons; we throw a few electrons at the atoms and by noticing how they respond, we get some idea about what sort of things atoms are. Electrons are our sticks, and they help...