Measure the page magnifier lengthwise. Subtract 1/4 inch from that number and use that measurement to draw a square on one side of the cardboard box using a pencil. Position the square as close to the center of the side as possible.
Cut out the square carefully with an X-Acto knife. Turn the box so it's laying down on your work surface on the side with the square cut out. Slide the page magnifier inside the box and lay it over the square hole with the grooved side facing down. Place masking tape along the edges of the magnifier where it overlaps with the cardboard and press down to make a seal.
Stand the box upright. Spread glue on each side of the inside of the box, except the side where the magnifier is, and glue down a single sheet of aluminum foil on each of the three sides.
Rip off the excess aluminum foil. Also, use scissors to cut off the flaps from the cardboard box.
Put sunglasses on your child and yourself. Go to an outdoor clearing at high noon and hold the box with the magnifier pointing up towards the sun. Find the perfect spot on the ground where the sun will catch the magnifier. You may have to hold the box and move it slightly around to catch the sun at the perfect angle.
Place a thick piece of wood inside the box underneath the magnifier. Move the box around until it catches the sun, hold it steadily in place and watch as the magnifier conducts the sun's energy into a powerful, and very hot, laser that will burn the wood.