Think about your lungs taking in and letting out air just like balloons. Try taking in and letting out air through your mouth and through your nose. To see what our lungs are doing, blow up several balloons. Tie a knot at the end to keep the air in the balloons. When our lungs fill up, they expand like the balloons and hold the air.
Air takes up the shape it gets into.
Look at the different sizes and shapes of balloons. Air took up the space and form that the balloons were. Blow up another balloon. Hold the end together with your finger tips pinching it. Slowly release your pinch to let air come out slowly.