Second, wind turbines also use important design elements of the modern airplane wing. The turbine's propeller blades are shaped like airplane wings. The wind pressure on an airplane wing is lower on the top and higher on the bottom. The difference in pressure causes the wing to lift and drag. While the airplane wing is stable, wind turbines are not: they are designed to rotate, so the action of lift and drag causes the blades to spin. The turbine blades are connected to a generator that produces electricity. In certain areas of the country, the winds are much stronger, and many of these large, wind-powered towers are erected on hillsides. When there are many grouped together like this, the area is called a wind plant