The dot at the end of this sentence is about as big as a water flea looks when we see him without a microscope.

He is indeed so small that when he jerks along in the water we cannot be sure that he has head, tail, or legs. In fact, we hardly notice him at all unless he moves. By looking at him through a microscope, however, we see that he is the fine animal that the picture shows; and now we can study his wonderful swimming legs, his queer tail, his rounded head, and his long, jointed feelers that reach around under his body.

Evidently, although our eyes are rather keen for some things, they cannot see everything. There are millions upon millions of tiny living creatures in the world that we cannot even catch a glimpse of without a magnifying glass.

Some of them are plants, some are animals, and we know most of them by what they do. They are so small that the water flea must look like a fearful giant to them, and perhaps they, in turn, seem to him quite too small to notice; or it may be that he is so much bigger than they are that it is as hard for him to see them as it is for us to see him. Whatever the case may be, and though they are so very small, still we know that some of them can do more harm than mad dogs, while others are more dangerous than whole armies of soldiers in the enemy's country.

They are around us everywhere,—in the air we breathe, in the water we drink, in the food we eat. At the same time they are of so many shapes and kinds that hundreds of men study them from one year's end to the other and learn strange facts about them. They also give them different names,—the yeast plant, the mold, and the bacteria. Some people speak of them all as germs, but the name that fits every kind best is microbe, meaning "small life."

Most microbes do us neither harm nor good. Multitudes of them are indeed the best friends we have, yet very many others are our worst enemies. It is the microbe that spoils our meat and sours our milk and gives us moldy bread. They rot the fruit after it is picked, and turn sweet grape juice into wine and vinegar. In all this they are so successful that we might call a piece of spoiled meat or fruit a microbe city, for millions of microbes are living there together and working as fast as possible. They are also ready to go from such a city to a new place, and there the new colony grows fast.

Take a sound apple and a rotten apple. Crack the skin of the sound one, press the rotten one against it, and leave them close together for a few days; then examine them and you will see that even the sound apple has begun to spoil. A few microbes promptly went across from the rotten to the sound apple and made their home there. After that nothing can save an apple except to cut off every bit that is spoiled; but even then it will not keep long, for the skin is broken and other microbes from the air can get into it easily.

This is true of every kind of fruit, and also of meat and milk, in fact, of anything that can decay. Microbes move fast and they start new homes in whatever is nearby. Cooks who know this are careful to protect their food from microbes if they can. They never put fresh meat near meat that is spoiling, nor new milk into a pitcher with old milk.

Cooks also know that microbes work fastest in warm places and slowest where it is cold, and for this reason, when they want milk to sour in a hurry, they put it in a warm place; when they want it to keep sweet, they put it in the ice chest. They know that it will surely keep if it is frozen. They also know that meat, if it is frozen solid, will keep long enough to be carried to the North Pole and back again, though on a warm summer day, in a hot kitchen, it may begin to spoil in a single afternoon.

To understand this we must know that microbes do not die when they are frozen. Instead, they can endure more cold than the strongest man. It is true, of course, that while they are frozen they can do no more work than a frozen man, but the difference is that when the freezing is over the man is dead, while the little microbe is as sprightly as ever and ready for business.

Since they are so plucky about cold, it seems as if they ought to endure great heat too. Yet this they cannot do, for fire, or water that is boiling hot, actually kills them. This then is our good fortune, for we manage to save some of our food by boiling the microbes that get into it.

There are two sides to the secret of keeping things from spoiling:

1. Kill the microbes.

2. Put the food where no microbes can get to it.

All this tells about our food,—how it spoils and how to keep it sweet. But you and I are far more important. We send microbes into our lungs every day with the air we breathe; we put them into our mouth with our food and our drink; we gather them on our hands and our face from anything we touch. Though many of these microbes do us good, some of them do us great harm. They may give us sore eyes, typhoid fever, consumption, or any one of a long list of other diseases that make us miserable, put us to bed, or kill us outright. They are the smallest and the strongest enemies we have, and we need to know how to protect ourselves against them.