Science Work

unit 18:air and water
air
Living things need a constant supply of oxygen and carbon dioxide in the air to survive.They also constantly give out carbon dioxide.Oxygen in the air never runs out,and the air is never completely filled with carbon dioxide clue to the oxygen and carbon dioxide cycle.
water
Water never runs out on the earth due to the water cycle.The water cycle is the continuous movement of water from the earth,to the sky and back to the earth.Movement of water in the water cycle requires neat from the sun to take place.