Measure and draw two triangles on an index card. Make them matching isosceles triangles, with two equal sides and one longer side. Cut them out with scissors.
Cut slits into each end of a drinking straw.
Slide the long side of one triangle into one of the slits, making a pointing arrow shape. Slide the other triangle into the other slit point first, so it is pointing the same direction as the first triangle and its long side makes a fin in the back. Your straw should look like an archery arrow.
Measure the straw to find its midpoint. Have a friend hold a pencil point-downward, and place the midpoint of the straw horizontally on top of the pencil's eraser. Pin the straw to the eraser.
Flick one end of the straw to make sure it spins easily. If it does not, loosen the pin.
Stand in a windy place and hold the pencil point-downward. Watch the straw spin to point in the direction of the wind.