Being too young to fly a jet plane was onerous to Cappy. But when he discovered that Timmy Brown's father was a jet pilot, the days weren't long enough to get answers to all the questions Timmy and Cappy asked about what makes the jet fly. The principles of simple engines with examples of physical laws are couched here in primer terms, to help young science enthusiasts to grow up with some understanding of the high powered aircraft which take Man's horizons to the moon. The explanations given really are lucid and helpful. The author likens the physical forces which impel the jet engine to things familiar to children: he equates balanced forces with a teeter-totter, finds examples of the principle of action-reaction in the sport of the two boys pumping a swing. A good appetite-whetter for more information later, on air travel.