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.
WHY ONIONS MAKE YOUR EYES WATER
Your eyes really "water" a little all the time. All of our special sense organs—the tongue, the nose, the ears and eyes are kept moist by special glands. They are exposed to the air and must be kept clean. Moisture probably makes the nerves of the sense organs more sensitive to impressions. The inside of our hands and fingers, where we feel most keenly, are moister than the backs. Now, when there is trouble in a sense organ, like too strong a taste in the mouth, or a "cold" in the nose, or a grain of dust on the eye-ball, the moistening fluid just gushes out to wash it away. Then the eyes or nose or mouth "waters" or overflows.
The eye is irritated by many things beside dust. The fumes of vinegar, pepper and onions make the eyeballs smart. Instantly the tear-glands overflow to protect the eyes. If they could not do this the eyes would become inflamed, and they might be permanently injured. Just watch now and see if onions and pepper do not make you sneeze, and make your nose "water," too, as well as your eyes. Think of lemons and feel your mouth "water."
WHY WE COUNT BY TENS
We count by tens because we have ten fingers—five on each hand. Nature is very fond of counting by fives. Five fingers on each hand; live toes on each foot; five petals in an apple blossom; five rays in a star-fish. In the Story of Life you will find a whole chapter on how nature counts in making the parts of plants and animals. When men became bright enough to want to count, they
