My Grade 7 son is designing a solar oven to melt s'mores for his school homework. We made one out of the medium-sized pizza box with aluminum foil, plastic wrap and construction paper (materials were limited to these). We left the pizza box oven indoor, next to a window, with full sunlight for 4.5 hours. The chocolate part melted easily, but the marshmallow turned hard and dry. Was the oven not hot enough to melt the marshmallow? (Unfortunately, I don't have a thermometer to check the temp inside the box.) Marshmallows used were store-bought, big and new, and we tried it with a whole and a halved marshmallow. It's March here in Toronto.