It is a fun & simple science experiment that kids can really get in to and enjoy.
Start by finding a box with lid (re-use a pizza box if you have one.) Be sure the box is large enough to fit the size or tortilla you will use. Cut a whole in the top of the box that is about the size of the tortilla.
Line the box (top & bottom) with aluminum foil.
Apply some plastic wrap over the cutout. This is optional, but it does keep out any dirt or critters that happen to be about.
Place the tortilla in the bottom of the box and spread pizza (or spaghetti) sauce over the entire tortilla. Sprinkle a thin layer of cheese.
Place cover over the top and place in a sunny spot. Periodically check on it to see if it is ready.
This was 2 hours after it had been set out to cook. It was a mostly sunny day that was 83 degrees. The cheese was mostly melted and she was happy with the amount it had cooked and wanted to try it.