What you’ll need

  • One small and one large cardboard box, in which the smaller box fits easily
  • Aluminum foil
  • Cling film
  • Baking paper
  • Adhesive tape
  • Newspaper
  • Black craft paper
  • Carpet knife
  • Scissors
  • A ruler
  • Wooden skewer
  • Chocolate
  • Optional: Bread

How to do it

1. Using the ruler and a pencil, draw a big square on the top of the small box.

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2. With the help of an adult, use the carpet knife to cut out the square on three sides so that you have an opening with a flap.

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3. Line the interior of the box, including the lid, with aluminum foil.

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4. Cover the base of the box with black paper.

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5. Cover the opening of the box with cling film: Attach it to the outer edges using the adhesive tape. Leave a narrow opening at the front of the box.

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6. Place some crumpled newspaper at the bottom of the larger box.

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7. Insert the first box into the second one and stuff more newspaper into the space between the two boxes.

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8. Place a piece of chocolate on the baking paper. Gently slide the baking paper with the chocolate on it through the opening and place it in the middle of the box.

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9. Place the boxes outside in the sun. Use a wooden skewer to prop the lid of the box open at an angle such that light is reflected into the box. Next, every five minutes check how the chocolate changes. At the end, you can enjoy the chocolate with a slice of bread, for example.

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Well spotted!

The chocolate melts after a while. If you have a thermometer you can watch how the temperature rises inside the box. The speed at which the temperature of the oven rises will depend on how well insulated the box is, the strength of the sun and the temperature of the surrounding air.

Example:
The experiment was conducted in bright sunshine at an outside temperature of 20°C. On the south-facing balcony, the temperature in the small box rose to 35°C (thermometer was in the shade). The chocolate (dark) melted within 10 minutes. Chocolate outside the oven did not melt.

What's the secret?

The aluminum foil reflects the sunlight and directs it to the interior of the box. The sunlight passes through the cling film into the box and warms the air inside. The black paper provides for additional warmth because it also absorbs the energy of the light and heats up. To ensure as little heat as possible is lost, the box is inserted inside a second box lined with newspaper, which provides insulation – just like when you put on a sweater to keep warm.