Card (cereal box), piece of paper, sticky tape.

Cut a Kite shape into the piece of card. The side of a cereal box is the perfect thickness and size. Take a A4 sheet of paper, fold one corner over to get a large triangle and cut it out. Fold the card and the paper triangle along the dotted lines shown on the animation. Tape the bottom (Long side) of the triangle to the top (Inside) of the card. It should stick up at 90 degrees to the card. Fold the card in half along the seam and when doing so push the two folds indicated by the dotted lines on the triangle so that they are flush against the card seam. Hold at the pointy end and flick you wrist forward to make a wicked cracking noise.