How does leading edge vortices LEV create lift I'm writing a text for school about samaras, seeds that spin to create lift. The wing of some of these seeds produce leading edge vortices that somehow generates lift, but I can't properly understand how. Are the vortices forcing passing air to pass above them and so creating a longer way for the air passing above the wing, thus creating a lift?
 A: Leading Edge Vortices essentially work by forcing air downwards. 
Vortices are trapped on the upper side of the wing. When air flows past, it is forced downwards, thus propelling the wing (and whatever is attached to it) upwards. Notably, super-sonic jets have used this a lot, like the Concord, as it allows for angles of attack that otherwise would have caused the plane to stall.
Here is an image of how a different kind of vortex (sadly couldn't find a good image for a leading edge vortex) looks in a plane:

In the case of your seed project, it would seem that vortices form on the upper part of the "propeller" on the seed, forcing air downwards and creating a bit of lift. 
A: Those little propellers are just wings, and the trick with any wing is to keep the flow attached to the top surface.
One way to do this is to energize the boundary layer by inducing vortices in it.
Here's a video on the subject.
A: As the image in this thread shows, creating a vortex is simply a matter of geometery and local flow patterns. In the case of a wingtip vortex you have low pressure on top and high pressure on the bottom, which allows the air on the bottom to flow around the tip into the low pressure on top. This causes a vortex.
The formation of the vortex causes a loss of lift on the underside because the air moves on top. This effect is reduced if the wing-tip is smaller front-to-back, which is one of the reasons aircraft that spend their time at lower speeds tend to have very long, skinny wings. Gliders are the canonical examples. Careful shaping also helps, which leads to designs like the Spitfire's elliptical layout, or its close cousin, the higher tapered wings seen on Soviet WWII aircraft.
The "trick" is that the vortex is self-stable, meaning it keeps its form even in the presence of other aerodynamic forces. This is why wing-tip vortexes are dangerous, they can still be powerful at long distances from heavy aircraft, even after that aircraft has landed. This was a serious problem in the 1960s when the first really large jets were arriving, and required major changes to air traffic control to avoid putting light aircraft in the stream, along with the call like "295 heavy" which reminds everyone what's going on.
That same behaviour can be used to your advantage. When the US was testing its first delta-wing aircraft, they noticed that it remained controllable at very high angles of attack, and Yeager managed to land it at something like 60 mph on one test. The reason was that the airflow at the interface of the fuselage and the wing was causing a vortex to form on top of the wing. The stability of this vortex meant it remained in roughly the same position over a wide variety of angles, keeping fast-moving air on top of the wing even in cases where it would have otherwise separated.
The modern LEX is really just a small delta wing. It is positioned so that in normal flight it is flat to the airflow, but when the aircraft "pulls up" it begins to generate a vortex and allows the angle to be increased. The downside, as in the wing-tip case, is that this also causes a large amount of drag, but perhaps less than a conventional wing able to generate the same amount of lift under the same conditions.
So... what does this have to do with a samara?
This is actually a very interesting case. Look closely at the seed and you'll notice it doesn't really have a very good low-speed wing shape. Ideally you'd want something more concave, and with a thicker leading edge. What appears to happen is that the circular motion of the seed, along with the shape of the inner section (where the actual seed is) causes a vortex to form. Two additional points: like the LEX, the vortex forms at an angle, and the "wing" is swept back. As a result, the vortex is formed along the leading edge (IIRC the spinning also has something to do with it).
What's happening is that the vortex is forming a sort of "virtual leading edge" along the wing, producing a shape that is much more aerodynamic than the wing would be otherwise. Now that is pretty cool. The exact same effect is seen in insects, bats and some birds.
