Here is a real spinning windmill made from just one plastic bottle and a thin length of bamboo.
First get an empty plastic drinks bottle. Any size from 500ml to 2L will be fine.
You need to cut three vanes into the bottle. Scissors and knives will cut bottle plastic with ease, but it is slippy so don't let the kids cut it themselves or you may find yourself taking an unplanned trip to your local A&E.
Cut each vane as shown (right) and then fold back along the dotted line. Make three of these flaps equally spaced around the bottle.
The trickiest bit comes next. You need to make a small hole in the middle of the bottle's base. This is where the plastic is thickest. Eventually I made the hole by heating a blade and meting a hole in the base. Make the hole big enough for one end of a thin bamboo stick to go all the way inside the bottle leaving a good metre sticking out the bottom end. The bottle lid will rest on the end of the Bamboo creating a pivot aroud which your windmill will spin in the breeze.
Next, Let the kids decorate the bottle. Shiny, flashy reflective materials are great. If the cut edges of the bottle seem sharp, then a bit of tape can act as a temporary safety device.
Now place the bamboo stick straight into the ground in your garden and drop the bottle on top. In the smallest wind the bottle will spin gently, entertaining the kids and keeping the birds off the cabbages.