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 condensing milk, the milk is heated in steam tanks to the boiling point, and kept there until most of the water passes off in vapor. Then it is sealed, boiling hot, in air-tight tins. Milk is a favorite food of many little plant and animal cells that harm us if we drink them. So, to keep as many of them out as possible, cows should be healthy, and stables and milkmen clean. All pails and strainers and bottles used should be boiled. The milk should go into the bottles as quickly as possible, be sealed up with waxed paper caps, cooled and shipped at once. The ice box must be kept clean, and the bottles sealed until the milk is wanted. Never allow milk to stand in a warm place, or in an open vessel. If you do, millions of microbes will move right in and begin to grow.

Guess why they are alike. They all sing. They all sing in much the same way, too.

Watch very close and find out how you sing. You fill your lungs with air. Then you nearly close your throat and force the air out through a small opening. As the air goes out, it presses on the cords in your voice box and sets them to trembling, or vibrating like violin strings. A bird sings in the same way. Now watch the kettle. The kettle is full of water and air. Don’t forget the air. When the water begins to boil it turns into vapor, or water gas, by exploding, into bubbles. Vapor needs more room than water. So the first thing the vapor in the kettle does is to force the air out through the spout of the kettle. That is just like the narrow opening in your throat. Then the vapor keeps coming out in such rapid little explosions that the kettle vibrates. That makes the singing. Sometimes it "sings," or vibrates, so forcibly, that the lid of the kettle dances, keeping time like castanets.

What is quite so wretched looking as a wet hen? She is drenched by a hard rain just as quickly as you are. But you never saw a wet duck, did you? Yet a duck spends most of its life swimming in ponds. It dives for food and comes up as dry as bone. The feathers do not account for this, for the duck's feathers are not so very different from the chicken's. The secret of it is that the duck's feathers are oiled. There is an oil-making gland on the duck's back near the