Screenshot by Tim Hornyak/CNET

How many times have you tried to switch on a flashlight, only to find the batteries are dead? Well, here's one that works simply with the heat of your hand. All you need is to be alive -- and warm.

The Hollow Flashlight is the invention of 15-year-old Ann Makosinski of Victoria, Canada, whom we wrote about in July. Her flashlight is one of four winning creations in the annual Google Science Fair.

With more than a thousand submissions, the event highlights teen innovation around the world. The grand prize of a $50,000 scholarship went to San Diego's Eric Chen for his use of computer modeling to discover new flu medicines.

Chen is only 17 and is already trying to save the world. He used a supercomputer and biological experimentation to find ways to speed up the discovery of influenza inhibitors ahead of future flu pandemics.

Fourteen-year-old Viney Kumar of Australia, meanwhile, won in the 13 to 14 age group for developing an Android app that warns drivers nearly 70 seconds before the approach of an ambulance or other emergency response vehicle.

Dubbed "PART," the app uses GPS data to give motorists plenty of time to get out of the way, reducing the possibility of responders getting stuck in traffic.

Turkey's Elif Bilgin, 16, was honored for a project in which she managed to create bioplastics from banana peels. It's aimed at reducing the use of plastics derived from petroleum.

Meanwhile, Makosinski wants to manufacture her flashlight with Peltier tiles, components that produce electricity when heated on one side and cooled on the other.

The LEDs are lit when body heat from a user's hand warms the exposed tiles, while ambient air in the flashlight cools the other side. Aside from experimenting with the body design, Makosinski had to modify the circuit to produce enough voltage to trigger the light.

If they can be made cheaply enough, the flashlights could help people in developing countries who don't have access to electricity or light in the evening.

Check out the explanation and demo of the Hollow Flashlight below.