The part of the vane that turns into the wind is usually shaped like an arrow. The other end is wide so it will catch the smallest breeze. Sometimes a metal rooster or other animal sits on top of the weather vane. If the wind is blowing from the south, the wind is usually warm. If the wind is blowing from the north, the wind is usually cooler.
Some weather vanes have directional strips underneath the arrow to make it easier to read.
Your markings on the bottle do the same thing. The breeze turns the arrow on the weather vane until it catches both sides of the wide equally. The arrow always points into the wind. It is easier to see how the energy from the wind moves your weather vane if it is up high and in an open area. You might also want to experiment by putting it on the ground.