Source : Free On-Line Dictionary of Computing

hairy ball
     
         A result in {topology} stating that a continuous
        {vector field} on a sphere is always zero somewhere.  The name
        comes from the fact that you can't flatten all the hair on a
        hairy ball, like a tennis ball, there will always be a tuft
        somewhere (where the tangential projection of the hair is
        zero).  An immediate corollary to this theorem is that for any
        {continuous map} f of the sphere into itself there is a point
        x such that f(x)=x or f(x) is the {antipode} of x.  Another
        corollary is that at any moment somewhere on the Earth there
        is no wind.
     
        (2002-01-07)