  Combing hair involves brushing away the topological tangles in a collective
curl. Using a combination of experiment and computation, we study this problem
that naturally links topology, geometry and mechanics. Observations show that
the dominant interactions in hair are those of a two-body nature, corresponding
to a braided homochiral double helix. Using this minimal model, we study the
detangling of an elastic double helix via a single stiff tine that moves along
it, leaving two untangled filaments in its wake. Our results quantify how the
forces of detangling are correlated with the magnitude and spatial extent of
the link density, a topological quantity, that propagates ahead of the tine.
This in turn provides a measure of the maximum characteristic length of a
single combing stroke, and thus the trade-offs between comfort, efficiency and
speed of combing in the many-body problem on a head of hair.
