Cooking food over a smoky fire is found everywhere in the world. It is often difficult to find wood for the fire. People who do not have wood must spend large amounts of money on cooking fuel. However, there is a much easier way to cook food using energy from the sun. Solar cookers, or ovens, have been used for centuries. A Swiss scientist made the first solar oven in 1767. Today, people are using solar cookers in many countries around the world. People use solar ovens to cook food and to heat drinking water to kill bacteria and other harmful organisms. There are three kinds of solar ovens. The first is a box cooker. It is designed with a special wall that shines or reflects sunlight into the box. Heat gets trapped under a piece of glass or plastic covering the top of the cooker. A box oven is effective for slow cooking of large amounts of food. The second kind of solar oven is a panel  cooker. It includes several flat walls, or panels, which directly reflect the sun's light onto the food. The food is inside a separate container of plastic or glass that traps heat energy. People can build panel cookers quickly and with very few supplies. They do not cost much. In Kenya, for example, panel cookers are being manufactured for just two dollars. The third kind of solar oven is a parabolic  cooker. It has rounded walls that aim sunlight directly into the bottom of the oven. Food cooks quickly in parabolic ovens. However, these cookers are hard to make. They must be re-aimed often to follow the sun. Parabolic cookers can also cause burns and eye injuries if they are not used correctly. You can make solar ovens from boxes or heavy paper. They will not catch fire. Paper burns at 232 degrees Celsius. A solar cooker never gets that hot. Solar ovens cook food at low temperatures over long periods of time. This permits people to leave food to cook while they do other things. To learn more about solar cooking, you can write to Solar Cookers International. The postal address is nineteen-nineteen Twenty-First Street, Sacramento, California, nine-five-eight-one-one, USA. Or you can visit the group's Internet Web site. The address is www.solarcooking.org. How long is the first solar cookers invented?
About 1767 years More than 300 years More than 250 years Less than 250 years