This science project helps demonstrate one of the oldest weather measurement tools. The simple weather vane determines wind direction. The pointed end of the arrow indicates the direction where the wind comes from, if its balance point is situated toward the point and not at the middle of the arrow. The large surface area of the tail, or blunt end, gives the wind a big spot to push around. The wind pushes against the arrow point and the tail at the same time with equal force, holding the entire arrow in one position, pointing straight into the oncoming wind.
Cut a 1-inch slit into each end of a plastic drinking straw with scissors. Straighten out a small paper clip and twist a small loop into the center of it. Set these aside.
Fold a 3-inch-by-5-inch index card in half so that it measures 2-1/2 inches wide by 3 inches tall. Fold it in half again so that it measures 1-1/2 inches wide by 2-1/2 inches tall. Hold the rectangle by the bottom folds and mark the center point of the top edge's width with a pencil. Cut from the lower right corner to the top center mark, and then from the left lower corner to the upper center mark to make a triangle. Secure the edges with a piece of clear tape. This is your arrowhead, which is the thickness of four cards and still folded at its base. The extra weight on this end will shift the arrow's center of balance toward the point of the arrowhead, making the weather vane work.
Fold another index card into four thicknesses as in Step 2. Cut out one of the quarters of the card to make the tail portion of the arrow.
Poke the wide end of the arrowhead into the slit at one end of the straw, and a narrow side of the rectangular tailpiece into the other slit, positioning it lengthwise. Secure the arrowhead and the tailpiece to the straw with small pieces of clear tape.
Find the balance point of the arrow by setting the straw on your finger. Move it back and forth and reposition it until the arrow roughly balances itself. This is the arrow's center of balance. Mark the spot and make a very small hole completely through the straw at that mark with the tip of a hobby knife.
Poke one end of the paper clip through the hole in the arrow shaft and slide it up snugly against the loop in the clip. Make sure that the arrow fits tightly around the clip so that it doesn't spin around. Stick the other end of the clip securely into the side of a newly sharpened pencil's eraser.
Poke the sharpened end of the pencil into the ground outside, about 1 to 2 inches deep. Wiggle it around a little to loosen the soil just enough to allow the pencil to turn around loosely in the hole. The breeze will move the weather vane around so that it points directly into the wind.