What we call wind is air flowing horizontally above the ground in some direction. It is the result of sunlight warming the Earth's surface more in one place than another. This unequal heating creates areas of high and low pressure in the atmosphere. Air has a natural tendency to flow from an area of high pressure to one of low pressure and this flow creates wind. A weather vane is a device used to show the wind's direction. Weather vanes can be very complicated or very simple. A simple one is easy to build and works just as well as a complicated one.
Use the ruler and scissors to measure and cut 1-inch long slits in each end of the plastic straw so they line up with each other.
Make a pointer by cutting a triangle from one 3-by-5 card with a base of about 2 inches.
Create a tail by cutting the other 3-by-5 card into a 4-inch square.
Slip the pointer and tail into the slits in the plastic straw. Use the clear tape to fasten them.
Add weight to the bottle by filling it with sand or salt.
Hold the straw so the tail and pointer are vertical and push the straight pin through the middle of the straw in line with them.
Stick the pin into the eraser and push the opposite end of the pencil into the sand or salt.
Use the marking pen to print "North," "East," "South" and "West" on the bottle. Work clockwise and space the words equally.
Take the weather vane outdoors and align the "North" on the weather vane with north on the magnetic compass.
If you don't have a magnetic compass, use a street map of your area to find north. Set up your weather vane away from houses or objects that might deflect the wind from its true direction.
Have an adult do the cutting if you don't have blunt scissors.