Air is all around us.It is around us as we talk and play, From the moment we were born , we are surrounded(围绕) by air. When we sit down ,air is also around us ,we live in air .
   All living things need air:living things cann't live without air.
   We can go without food or water for a few days ,but we can not live for more than a few minutes without air. When we are working or running ,we need some air. So we breathe faster than usual. When we are asleep, we need less air.