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Trap the Sun’s energy in this DIY solar oven and heat up a s’more-licious treat!

This is an activity best done on a hot sunny day around lunch time when the Sun is overhead.

Safety: This activity involves using a box cutter knife. Ask an adult for help. Protect yourself from the sun by wearing a hat, shirt, sunglasses and sunscreen.

You will need

• Cardboard pizza box
• Ruler
• Marker pen
• Box cutter knife
• PVA craft glue or glue stick
• Aluminium foil
• Black paper or card
• Clear plastic cling film
• Tape
• Your choice of plain biscuits
• Marshmallows
• Chocolate (a brand which has thin pieces is best)
• Thermometer (not required)

What to do

1. Using your pen and ruler mark a square on the lid of the box, about 4 or 5 cm in from the edge.
2. With an adult’s help, cut along 3 sides of the square.
3. Score the 4th side of the square with the back of your pen and bend the card along the line, making a flap in the top of your pizza box.
4. Measure and cut a piece of aluminium foil the same size as your flap.
5. Now cover the inside of the flap with glue and stick the sheet of aluminium foil onto the inside surface of the window. Ensure the shiny side is facing out and that it is nice and flat, so that the foil is as reflective as possible.
6. Repeat steps 4 and 5 on the other surfaces inside your pizza box. Some areas might be too wide for a single piece of foil to cover, so you’ll need to use multiple pieces.
7. Make a big square of black paper and glue it to the base inside the oven. Depending on the size of your pizza box, you may need a couple of pieces of black paper to make the square.
8. To trap the heat inside the oven you will need clingwrap in your window. Tape a layer of cling wrap over the opening you previously cut in the lid, ensuring that it is airtight. If your clingwrap is too flimsy you can add a second layer, but be careful not to wrinkle it too much when you stick the 2 layers together.
9. Position the oven in a sunny, sheltered spot in the garden, and face the opening directly towards the Sun.
10. Before you start cooking, remember to wash your hands!
11. Place a biscuit onto the black cardboard in your oven. Put a marshmallow on the biscuit, and then add a piece of chocolate on top. You may wish to make several at once to share with your family or friends. If you have a thermometer place it next to the s’mores so you can read it through the window. Carefully shut the lid and prop open the flap using the ruler.
12. The s’more may take 30-60 minutes to fully melt, depending on how hot the day is. We did it on a 28 degree day and our thermometer reached 70 degrees Celsius and took 30 minutes for the marshmallow to fully melt. If it is taking a while, you may need to come back every 15 minutes or so and reposition the oven directly towards the Sun.
13. When you think your treat is sufficiently melted, open the lid of the box and, being careful not to burn yourself, place a second biscuit on the top. Press down gently to squish the marshmallow and chocolate filing together. Yum!

What’s happening?

There’s a lot of energy in sunlight. This oven is designed to put that energy to work making delicious treats! There are several tricks to make it work.

Sunlight enters the oven through the window. Extra light is reflected off the foil on the lid and redirected into the oven as well. Inside the oven, the black surface is very good at turning light into heat. Then the black paper radiates this heat, warming the air inside the oven. Normally, this hot air would rise and float away, but it’s trapped by clear plastic, so it keeps getting hotter and hotter!

This oven is slow to heat and it doesn’t get nearly as hot as an electric or gas stove. But it is completely renewable, powered only by the Sun and not by fossil fuels.

You might like to experiment to make your solar oven more efficient. You could try insulating the box with scrunched up newspaper, or block up any gaps in the box to stop heat escaping. You could add a second layer of clear plastic to the window leaving an air-gap between the layers to act as double glazing. You could add extra mirrors to shine even more light into the oven too!

If s’mores are too sweet for you, you might be able to melt cheese on bread, or heat a hot dog, but these are both a bit harder to to than melting chocolate. Don’t try to cook raw meat or eggs – this oven doesn’t get hot enough to make those ingredients safe to eat.