Or, you may explain that oxygen is necessary for respiration in animals. Organisms need oxygen to release the energy from their food. Oxygen is a gas that supports burning. Also tie in that nitrogen gas is used to make living matter. Plants and animals do not use nitrogen directly from the atmosphere.
Plants obtain it from the soil.
Animals obtain it from eating plants. Discuss the concept of the layers of the earth's atmosphere by showing a model of the atmosphere on the overhead projector or by drawing a model on the chalkboard. Relate to them that the earth is surrounded by a blanket of air that extends 1000 km or 100 miles above its surface. The air molecules are trapped by the pull of gravity from the earth's surface. Therefore the air pressure on earth is greater.