MOTHER: Here is a picture of some kinds. You see they are not all alike. Some are round, others are flat, or narrow, or long, or short; so you see they are of all shapes and sizes. Some are so very tiny it would take three or four thousand to make a row an inch in length. Others are large enough so we can almost see them without a glass. Some have no color at all; others are light colored, and some are quite black. There are millions of cells in one drop of blood. Your skin seems like one piece, yet it, too, is made of layers of cells. If we should look through a strong glass at a tiny piece of potato, wheat, and some oatmeal, we would find they are all made of cells. PERCY: And do the cells last as long as we live, mother? [Illustration: _CELLS_] MOTHER: No, they keep changing all the time. When we walk, run, talk, think, or do anything, some of these cells die, and others take their places. The new ones are just like the old; for if they were not, our appearance would so change that our best friends would not know us. While boys and girls are growing, they are putting many new cells into the house they live in. This is the reason auntie said the other day that she hardly knew you when she had not seen you for a year. AMY: What are the cells made of, mother? MOTHER: They are made of the food we eat. This shows we should be careful to put the very best things we can get into our body-building--I mean such as the body can use, for what we _like_ best is not always what is needed to build up and strengthen us. When you get hungry, that is the call of the body for food to make more cells, just as the mason calls, “More mortar,” or, “More brick,” so he can build his wall higher and stronger. If his mortar has but little lime, or is badly mixed, or he has only broken, badly-shaped brick, the wall will not be strong or beautiful. So if we give the body wrong kinds of food, it can not build such a house as you and I wish to live in. HELEN: If moving about kills the cells, will they live longer if we keep still? MOTHER: No, they are made to live just so long, and will die anyway. If we should not work or play, the dead cells would stay in the body, and make no end of mischief; but when we move about, it helps to carry them away, and new ones take their places. So you need not be afraid to run and jump, play and work; for the cells will take care to keep the house you live in all right, if you only give them the right kind of food, and not too much of it. OUR FRAME