Why doesn't sails go backwards when going up wind?

Question

Why doesn't sails go backwards when going up wind?

Answer 1

Sailboats do not actually go against the wind using their sails.  They tack at an angle in a zig zag method so that they use the sails to catch the wind and push them in an angle in the general direction of the wind.  

This takes a much longer time to get anywhere, that is why sailing ships took so much time and why we do not use sailing ships for much more than pleasure today.

Answer 2

The air pressure on the windward side of the sail is greater than the pressure of the leeward side.