Bang. Your car hits something in front, decelerating (defined) fast enough to trigger the air bag sensors. The sensors turn a switch that energizes a wire, sending electricity into a heating element in the propellant (defined), causing it to oxidize rapidly. This chemical reaction produces a gas that quickly fills the cloth air bag. As the gas expands, it cools considerably, as predicted by Charles' law (defined). As soon as the bag fully inflates, it starts deflating, cushioning the impact.

In less than 1/20 of a second, the bag inflates.

Then what happens?
Ideally, the rider does not strike the air bag until it's fully inflated, explains Dr. Murray Katcher, chair of the American Academy of Pediatrics Committee on Injury and Poison Prevention. "If you are in the way when it's opening, that's when you get into trouble." The reason? First, the bag is still moving toward you, increasing the closing speed (defined) between you and it. Second, as we just explained, the bag immediately starts to deflate, making it a better cushion. (By 0.3 seconds after inflation, the bag is supposed to be empty.)

That makes two separate reasons why unbelted passengers, or people who are too small for the seat belt design, are more likely to be injured.

But there's another source of problems: being struck in the face or neck. Air bags are designed for the upper 95 percent of women, and the lower 95 percent of men (in terms of body size). But children and smaller adults can take a hit in the face from a bag designed to hit an adult in the chest.

Why does the deployment sometimes leave a chalky residue? Because bag-makers frequently use chalk to lubricate the bag so it can expand quickly and smoothly.

Enough status quo. What are some bright ideas for making air bags safer?