My daughter had a pizza at Subway for lunch and wanted to keep the box for a project. She asked me what she should make and I suggested she try making the pizza box solar cooker.
She lined her box with black construction paper, black absorbs light and helps keep the heat energy from bouncing off.
She cut a square out of the lid and cut a slightly larger square out of a plastic container to make a window.
She wrapped the cardboard square she had cut out with tin foil and taped it in place. This reflects the sunlight into the container. She used a wooden chop stick to hold it at the right angle.
You can adjust the flap to get the light in the right place. If you make your head shadow the box and you can see the marshmallows reflected in the foil, your angle is about right.
The sun soon melted the chocolate. We had gone inside to watch a show and wait, and when we looked back the wind had taken our box and opened it. Our marshmallows were not too melted, but I think they had cooled off since the box had opened. They were still yummy, just a bit hard to eat. I suggest mini marshmallows for this size oven since the large ones touched the top of the box.
Plates would have been helpful. Well now we know for next time, and we still had fun.