Julie Murray, age 9, of Tucson, Arizona, for her question:

Is the amount of oxygen in the air always the same?

The amount of oxygen in the fresh air outdoors does not change very much. It is given to us by the green plants and the great forests. And the breezy winds blow it around and around the world, day and night, summer and winter. In the world out¬doors, there is plenty of oxygen for all the people and all the animals. There also is plenty for the plants, for they need to take back some of the precious oxygen they make.

But outdoors is not indoors. When tae get shut up in a room, there is only so much air with only so much oxygen. The more oxygen tae breathe, the less there is left  ¬and the room gets stuffy. This is why we have windows to let in supplies of fresh air from outdoors. This is why we make sure to open a window when we go to bed. After all, we go on breathing, even when we fall asleep. If all the windows are shut, the bedroom air gets stuffy and we wake up feeling weary and dreary.