On the top of the pizza box, use a ruler and a marker to draw lines parallel to three sides of the box, 1" in from each edge. Use a box cutter or sharp scissors to cut along the three lines, to make a flap in the lid of a pizza box. Fold this flap up so it stands when the box lid is closed.
Cut aluminum foil to cover the inner side of the flap. Glue the aluminum foil to the inner side of the flap, shiny side out.
Use two sheets of clear plastic wrap to create an airtight window for sunlight to enter the box. Do this by opening the box and taping one sheet of plastic wrap across the inside of the opening you made when you cut the flap in the lid. Then, close the box and tape the second sheet of plastic wrap across the same opening, on the outside. Tape very securely, to seal out air.
Glue aluminum foil to the bottom of your box.
Cover the aluminum foil with black construction paper (black absorbs heat). The black surface is where your food will be set to cook.
Take the box outside to a sunny spot. Adjust the flap to get the most sunlight possible to reflect off the aluminum foil and onto the plastic-covered window. Use a ruler, stick or large straw to prop the flap where you want it. You can also tilt the box, by rolling a towel and placing it underneath the box. (Hint: You can preheat the oven as high as 200 degrees Fahrenheit by setting it in direct sunlight for 30 minutes.)
You can make toast of a buttered slice of bread, or melt cheese on bread, tortillas, or chips. You can cook a hot dog, make nachos, or cook s'mores. To keep the paper liner clean, place items on a clear plastic or glass plate (a pie plate works well). To remove food from the oven, lift the lid of the pizza box, and use oven mitts or potholders to lift the glass dish out.