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It is not possible to comb all the hairs on a tennis ball in the same direction. This mathematical problem is known as Henri Poincaré's theorem. It was formulated at the end of the 19th century and can be mathematically described as "there is no non-disappearing continuous tangent vector field in even-dimensional n-spheres". However, it is most easily expressed as "you can't comb a furry ball without creating a quiff". This theorem was confirmed by Brouwer in 1912, and it has an interesting consequence: on an ideal spherical planet, there is at least one point where the wind is blowing. The planet doesn't have to be perfectly spherical, but it should be continuous - meaning it shouldn't have holes or bumps in the middle.
