Air

In the school holidays, Bhoomi and Surya visit their Ajji (grandmother) in a village at the edge of a forest in the Western Ghats. Ajji says the place is full of nature's treasures: pure air, fertile soil, plenty of sunlight, and trees that give food and shelter to animals. Without these treasures, no life on Earth would be possible.

One morning Ajji does breathing exercises, taking deep breaths in and letting them out, and the children join her. Try holding your breath: it is hard to hold it for long. The air we breathe in has oxygen, and our body needs oxygen to do its work. Hold your breath too long and your body does not get enough. We can live without food or water for a few days, but not without oxygen for even a few minutes. Most other living beings need oxygen too.

You cannot see air, but you can see what it does: the pages of a book flutter, and clothes on a line swing. Air is a mixture of gases. Your book shows it as 100 squares:

• nitrogen fills 78 squares;
• oxygen fills 21;
• just 1 square is for argon, carbon dioxide and other gases together.

A percentage is the number of parts in 100, written with the sign %. So air is 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide and other gases. Check: 78 + 21 + 1 = 100.