When you brought the comb near the hair, the hair had fewer electrons than the comb. Whenever one object has more electrons on it than another, the two objects are pulled toward each other; so there was an attraction between the comb and the hair, and the hair came over to the comb. As soon as it touched the comb, some of the extra electrons jumped from the comb to the hair. The electrons could not get off the hair easily, so they stayed there. Electrons repel each other--drive each other away. So when you had a number of electrons on the end of the comb and a number on the end of the hair, they pushed each other away, and the hair flew from the comb. But when you pinched the hair, the electrons could get off it to your moist hand, which lets electricity through it fairly easily. Then the comb had extra electrons on it and the hair did not; so the comb pulled the hair over toward it again. When you brought the charged comb near your ear, some of the electrons on the comb pushed the others off to your ear, and you heard them snap as they rushed through the air, making it vibrate. HOW LIGHTNING AND THUNDER ARE CAUSED. In thunderstorms the strong currents of rising air blow some of the forming raindrops in the clouds into bits of spray. The tinier droplets get more than their share of electrons when this happens and are carried on up to higher clouds. In this way clouds become charged with electricity. One cloud has on it many more electrons than another cloud that is made, perhaps, of lower, larger droplets. The electricity leaps from the cloud that has the greater number of electrons to the cloud that has the less number, or it leaps from the heavily charged cloud down to a tree or house or the ground. You see the electricity leap and call it _lightning_. Much more leaps, however, than leaped from the comb to your ear, and so it makes a very much louder snap. The snap is caused in this way: As the electric spark leaps through the air, it leaves an empty space or vacuum immediately behind it. The air from all sides rushes into the vacuum and collides there; then it bounces back. This again leaves a partial vacuum; so the air rushes in once more, coming from all sides at once, and again bounces back. This starts the air vibrations which we call _sound_. Then the sound is echoed from cloud to cloud and from the clouds to the earth and back again, and we call it _thunder_.