During the pilot's intentional stall maneuver, the glider inadvertently entered a spin from which he could not recover prior to impact with a lake. While in a left turn of approximately 15 to 20 degrees of bank, the pilot configured the glider into a stall in an effort to demonstrate the maneuver to the passenger. The glider stalled with both the nose and left wing dropping. Despite the pilot's attempts, the glider did not recover from the stall and subsequently entered a spin. Attempting to recover from the inadvertent spin, the pilot applied a full-forward stick input and full rudder deflection opposite the direction of rotation. The pilot's recovery efforts were unsuccessful and the glider impacted a lake. The fuselage floated long enough for the pilot and passenger to get out and swim to shore. The glider then sank in about 200 feet of water. The pilot did not report any preimpact mechanical malfunctions or failures.