Daniel was playing tennis when he noticed a new red box on the wall at the courts.

It said AED on it. He had no idea what it was.

Turns out it might be one of the most important things he has ever walked past.

An AED -- Automated External Defibrillator -- is a device designed to help restore a normal heartbeat during a cardiac arrest. It sits in a box on a wall at a tennis court, an airport, a school, a shopping center, a gym. Most people walk past it every day without knowing what it does or that they are allowed to use it.

Here is what most people get wrong. During a cardiac arrest, the heart has not simply stopped. In most cases it has gone into a kind of electrical chaos -- quivering instead of beating, sending out confused signals, not pumping blood at all. What an AED does is deliver one precise electric shock that r ...