You’re right, it goes against all common sense. Yet it happens. That’s why, when sailing, it becomes necessary to mentally throw out much of what you know about the wind.
Forget paper blowing down the street. Forget leaves blowing across the surface of a pond.
Sailboats don’t work that way. If they did, the only way to use one would be to turn the sail sideways and let the wind blow you in the same direction. Instead, a sail works more like an airplane wing. The result is that the wind actually sucks the boat forward instead of blowing it backward.
Here’s how it works. The billowing shape of the sail makes the wind on the outside curve of the sail flow farther and faster than that on the inside curve.
Because of that rapid movement, the air molecules get spaced farther apart, creating a suction that’s twice as strong as that on the inside curve, sucking the boat in that direction.
This suction by itself would try to pull the boat sideways. Luckily, the keel (that board sticking out of the bottom of the boat) keeps the boat moving forward, and the skipper keeps the boat from tipping over by leaning out from the other side of the boat.
No sailboat can sail directly into the wind, but it can come pretty darned close, sailing at an angle only 12 to 15 degrees off from the line of the wind.
To go directly into the wind requires “tacking” (zigzagging back and forth).