A spirometer is an instrument that measures how well your child's lungs are working. To use one, your child breathes into a mouthpiece. The doctor may ask your child to breathe normally or to take a deep breath and blow the air quickly out of his or her lungs, like blowing up a balloon. A recording device measures the amount of air inhaled or exhaled and how much time each breath took. These measurements can help the doctor find out if your child has a lung problem, such as asthma.