Kathleen Mantel, age 9, of Nobleford, Alberta, Canada, for her question:

How many times does an adult breathe a day?

We all breathe faster when we exercise, slower when we rest and slowest when we sleep. As people grow up, they tend to take larger breaths and not so many of them. When sleeping, an adult may take six, seven or eight breaths a minute. During an usual day, a grown up person takes about 16 breaths a minute. This adds up to almost 19,000 ins and outs during a usual day and night.

We breathe the oxygen from the air. And the body regulates how much it takes in to how much it needs. Adult lungs hold about four quarts of air, but most of the time they do not empty and fill them with every breath. Each time they breathe out about a pint of stale air and breathe in a pint of fresh air. Athletes use up more oxygen and their lungs work harder. They may use up four quarts of air with every breath.