How do aircraft stall warning systems work?
Audible Warning
On light aircraft there is a reed (much like used on a musical wind instrument) mounted on one wing root, which is angled such that at the Angle of Attack which would cause a stall, the reed &quot;plays&quot; which can be heard in the cockpit.
Here is a view of where this system is mounted on a Cessna:

(source: weekendcfii.com)
On some aircraft, it is a similar principle; however instead of a reed, it uses a fin which at critical AoA pushes a micro-switch which activates a buzzer/horn inside the cockpit.
Here is the assembly on a Beech 18:

(source: eaa1000.av.org)

So, if you ever see anyone doing this:

They are not kissing their aircraft for good luck, merely testing the stall warn as part of pre-flight checks!
Tactile Warning
In addition to an audible reed/buzz/horn, most aircraft also provide tactile feedback to the pilot of an impending stall. In purely mechanical aircraft the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot when approaching a stall. In hydraulic or fly-by-wire aircraft, this is provided by a mechanical &quot;stick shaker&quot; linked to the same system as sounding the audible alarm.