Hm! Hm! Let's see. A bee isn't the only thing that hums. A wheel, turning rapidly, whirs. A teakettle "sings" when the water boils. A violin string quivers with a musical note that slowly dies away. A tuning-fork hums. A fly buzzes. All these humming, singing, whirring, buzzing sounds are made in the same way—by vibrations, or regular, rapid motions. The bee moves its wings very rapidly and regularly, when it flies—ever so many times in a second. This vibration, or trembling, sets up air waves. As you learned in the story of the telephone, sound travels on these air-waves. We can hear these sounds if they are loud enough. If the vibrations are very close together they make a continuous sound. Bird's wings beat the air, making a much louder sound than the bee, but the beats are so far apart that the sound of one dies away before the next comes. The humming bird's wings vibrate as rapidly as a bee's, so it hums like a very big bumble-bee.