Observations and results
When you blew straight onto the front of the pinwheel did it spin counterclockwise? Did it spin best when you were blowing into its "cups"?
Most pinwheels have the blades arranged so that when wind blows straight at them, they spin counterclockwise. This is because the blades' "cups" are made so that the oncoming air is captured and pushes the blades in this direction. (You can try looking at several different pinwheels to see this pattern shared among them.) When wind is blown into the cups, the pinwheel spins well. Consequently, if you are using a typical pinwheel, turn it so that the front side is facing your right and blow into the side of the blades. The blades will spin quickly counterclockwise if you blow on the bottom half (into the cups) but will spin slowly clockwise if you blow on the top half (against the backs of the cups). Similarly, if the pinwheel's front side is facing your left and you blow into the side of the blades, they will spin quickly counterclockwise if you blow on the top half (into the cups) but will spin slowly clockwise if you blow on the bottom half (against the cups' backs). If your pinwheel were a wind turbine, and spinning counterclockwise turned wind energy into electrical energy, then it would most efficiently produce electricity when wind blows directly into the cups.
More to explore
How Do Wind Turbines Work? from the Office of Energy Efficiency and Renewable Energy
Wonders of Wind (pdf), from the NEED Project Contributors
Fun, Science Activities for You and Your Family, from Science Buddies
Unleash the Power of a Pinwheel, from Science Buddies