Before sails, people would just float in the water on rafts or boats. The rafts might have animal skins filled with air under them. Sometimes they would use their hands or a stick to help steer or move the boat.

The first sail was invented more than 5000 years ago. It was a square piece of cloth hung on a stick at the front of a boat. The boat was now able to travel using the power of the wind. Since ancient boats did not have a keel (to keep it going straight) the wind had to be going in the same direction the person wanted to go (downwind)!

Sails became bigger, but they were still square for several thousand years. In the 9th century A.D. someone invented the triangular sail. Usually there were two sails, one in front of the mast (a pole) and one behind. This kind of sail could use the wind from almost any direction. The boat, with a triangular sail, could even move against the wind! These boats were much faster and could move in more directions, not just downwind.

To help understand how this is done think about a triangular sail. If the sail is pointing in the right direction the wind fills the sail. The sail pushes out toward the front. This makes the wind move faster around the front of the sail. The fast moving air creates a low pressure area in front of the sail.

The "push" of the wind behind the sail and the "pull" of the low pressure in front of the sail makes the boat move forward. The most important thing is for the sail to be set in the right place. If the sail is not in the right place, the sail will not fill with air and the boat will not move.

Most of the time the strongest air pressure is on the side of the boat. The keel (a wooden or metal piece sticking out of the bottom of the boat) keeps the boat from moving sideways. The boat will continue to move forward.

Suppose you hold a mop handle straight up from the floor and push down - there is no movement. But if you lean the mop handle to the side, just a little, and push again with the same amount of force, the mop slides easily across the floor. This is what happens if the sail's angle is changed even a little, in the right direction. Because of this a boat can go faster against the wind than it can going with the wind!

Airflow around a sail is very similar to airflow around an airplane wing.