By watching sunspots astronomers can tell that the Sun takes about 25 days to rotate at the equator and about 35 days at the poles. Each of these sunspots creates a magnetic field. As the sunspots move the magnetic fields move. Because of the different rotation times the magnetic fields eventually get so tangled up that they ‘rearrange themselves’ and this is what creates the 11 year sunspot cycle.

(The Great Courses – A Visual Guide to the Universe)