Here is what you need: Pizza box; black construction paper; aluminum foil; saran wrap; hot dogs; toothpicks; tape; glue
Doing it: Open up a clean pizza box. The box will have a top flap, and a bottom box. Line the bottom box with black construction paper and glue it down. Cover the top flap with aluminum foil and glue it on as flat as possible. Bumps or irregularities in the foil will reduce the ability of the top flap to collect sunlight. Poke two toothpicks into each end of a hot dog to make a little stand. Place the hot dogs in the box, and cover it with saran wrap. Make the saran wrap as smooth as possible too. Put the box in a sunny place, and adjust the top flap so that the sun is being reflected straight into the box. You may need to prop up the flap. I used two wooden skewers and two clothes pins to prop up the box.
Explanation: The sun is the ultimate source of almost all the energy on this planet. Black materials absorb light (which makes them black) and then have to get rid of all that energy they absorbed, which they do by heating up. The black construction paper heats up, and so heats the air all around it. The air can't escape the box because of the saran wrap, and so just gets hotter and hotter, cooking your food. A well built solar oven can reach far hotter temperatures, by making the solar reflector out of mirror instead of foil, by placing insulation around the box, and by using glass instead of saran warp.
Note: How does this help reduce using energy? Obviously, it eliminates the use of energy in an oven run by electricity or gas, but it also helps reduce energy usage in other parts of the home as well. Air conditioning on hot summer days is a massive energy drain, and those days are exactly when solar cooking could work, instead of heating your house with an oven only to cool it down again. Also, the oven is often placed right beside the fridge! The solar oven is an example of passive solar heating, which is using the energy of the sun to heat things up directly. Well designed houses can be heated by entirely passive solar, except during the darkest, coldest months of winter.