It is a loss of relative airflow over one wing causing a loss of lift. The controls become ineffective and your pants become full

Sometimes it's just down to the girth of your passenger

TSC.

Why do you think I always sit in the middle?

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Which usually happens when one wing stalls before another, like a stall in a climbing-turn, or in a cross-controlled situation.

Depends on the type of aircraft. Some aircraft tend to drop a wing at the point of stall. This can sometimes be quite violent & easily lead to a spin if not immediately corrected.* There are all sorts of aerodynamic devices to prevent this.

*PS. One classic example is the Bf 109.

I think the question is more along the lines of: why do airplanes continue to spin ("why is this a stable state?").

And I agree that there's no simple answer- planes exhibit a staggering variety of behaviors in a stalled state, and there are many kinds of spins.

But the key to understanding it is to remember that at that point, the shape of the airfoils doesn't matter much, because the airplane is no longer flying... it's falling. I know a plane can stall "on the upline" and continue to climb (briefly)while spinning, and of course a snap-roll is nothing more than a "horizontal spin", but it's not due to any lifting action of the wings, it's due to momentum.

It's more of a ballistics question than a flight question.

The action of the rudder in reducing or cancelling the spin so that the airfoils and stabs can get the airplane flying again is more like the action of the control surfaces on a rocket or bomb, although generally speaking all the rudder does is move the nose back and forth anyway, even when the plane is flying.

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You make it inherently stable. There are various ways of doing this from using a large dihedral angle to washout on the wingtips. A high-wing type is usually more stable than other types due to the pendulum effect of the low centre of gravity. The shape of the wing is also important & a sharply tapered wing will usually suffer from tip-stall problems.

There are various devices that make the inboard part of the wing nearest the fuselage stall before the tip like the automatic wing slats (used on aircraft like the Bf 109 & Tiger Moth) & small fixed leading edge spoilers as used on the DHC Chipmunk. These are usually fitted to reduce any tip-stall tendencies discovered during initial flight testing of a new type. Some aircraft have been designed from the outset to be impossible (or very difficult) to stall or spin but in most cases this affects performance. If they do happen to spin I wonder how easy it is to recover. One example is the highly efficient wing on the Cirrus SR20/22 series where the recommended spin recovery procedure is to deploy the emergency safety parachute. This will almost certainly damage the aircraft & possibly injure the occupants so the answer is to avoid spinning it if at all possible.