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 Sunlight, falling through the negative, bleaches the paper back of it to a true picture of the original subject. A photograph is made in the same way as the picture in your eye. Both are made by the light of the sun.

If you do—crack! You may break it. If the glass is thick it is almost sure to crack. The thinner the glass the smaller the risk. That’s odd, isn't it? Why is this? Heat expands. When a thing expands it needs more room. Glass expands readily. If it is thin the heat goes through quickly, expanding all parts alike. But when the glass is thick, the inside particles expand before the outside becomes heated. So the cold outside layer has to crack to give the warm inside layer room to expand. The same thing happens, sometimes, in pouring hot coffee into a cold cup. If you put a silver spoon in your glass or cup it will be less liable to be cracked. The metal attracts the first heat, and allows the glass or china time to heat more slowly and evenly. And a glass or china cup is apt to be cracked if it is very hot and you put ice water in it. Cold contracts or shrinks the glass. So the inside shrinks while the outside is still stretched.

We need salt in our bodies to keep us healthy. Our blood is just about as salty as sea-water. Isn't that curious when we think that animal life began on our earth in the sea? Now salt melts or dissolves easily in water, and in our blood, until they become brine. If we eat salty food the body soon gets too much salt in it. The only way to get rid of it is to dilute it with water. When the body needs water it very promptly calls for it. We say we feel thirsty. Salt isn't the only thing that makes us thirsty. Sugar, and many strong, hot spices heat the blood and need to be diluted with water and washed out.

An engineer would tell you that a locomotive is driven by steam power. He understands what that means, but you don't. So that is no answer for you. You know what steam is, of course. It is vapor, or water turned to a gas by heat. In turning into gas it expands, or takes up more room. What a fuss steam makes to get out of a teakettle! If you cork the spout, the steam pushes up the