Answer:
Sails catch the wind which propels the boat forward
2nd Answereer says:
Previous answerer has been tricked by a trick question:
IN REALITY the wind foils around the the sail and PULLS the boat forward. A sail is nothing more than a vertical wing, providing LIFT. Should you ever have the pleasure to experience a strong puff of wind startling your becalmed craft, you will FEEL the LIFT that is provided. It's a physics thing (Aerodynamics).