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A great many boys will think they know the answer to that. They will say it is because of the rim, or flange, that is on the inside edge of the car wheel. That rim merely steadies the car—keeps it running smoothly. But notice the next time you go to the railway station, that the rail is not flat on top. It is slightly rounded. If a freight car is standing on a side track you will see that the rim of the car wheel is not flat, either, and it does not rest squarely on the top of the rail. Beside having a ring-like flange on the inner edge, the broad rim of the wheel slants much like a cork that is made smaller at one end so it can be pushed only part way into a bottle. The outer circumference of a car wheel is smaller than the inner. And the wheel rests on the inner slope of the rounded rail.

In running, the wheels press outward. That is, the wheels opposite each other push outward or away from each other. This is because it is easier for them to run along the smallest edge, and on top of the rail. By pushing outward, with the same force in opposite directions, they keep each other balanced and on the rails.

You know milk turns sour when it is a day old, sometimes. But you can buy milk that is a year old, or more, that is just as sweet as it was when it came from the cow. It is condensed, or evaporated milk, sealed in air-tight tins. When the can is opened the milk in it will soon turn sour. That is because it is exposed to the air. The air is full of microbes, or little living plant cells like yeast and mould. They are too small for you to see, Like all plants they need soil to grow in. The soil they like best is a liquid with sugar or starch in it like milk, fruit juices and flour batters. In growing, these tiny plants change sugar and starch to acids. So milk becomes sour and fruit juices ferment. After a time the fruit juices stop fermenting and turn to vinegar or wine. But because milk is an animal product it decays.

If milk is cooled quickly, and kept on ice, the microbes find it much harder to live and multiply in it. Iced milk can be kept sweet for two or three days. It can be kept still longer if it is first heated in a corked bottle to a temperature that kills the microbes, then cooled quickly. This is called Pasteurizing. In cities, people have to Pasteurize milk to make it safe for babies. In evaporating, or