Step 1Cut out cardboard partas to match your bedroom window.
In the lower part of a cardboard, cut a slot about 4 cm high and 1 cm wide.
Darken the room by fixing the cardboard on the window with adhesive tape (if there is another window in the room, close its blind or shutters).
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Step 2Fill the freezer bag halfway with water and close it tightly.
Tape the bag so that the water surface is at the bottom of the slot, but not below.
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