A series of demonstration flights had been arranged by the aircraft owner. The weather was clear and sunny, with occasional wind gusts up to about 5 knots. Thermal activity was reported in the area. The pilot was the manufacturer of the aircraft, which was sold in kit form. Although he had considerable experience on the aircraft type, he was not familiar with flight in thermalling conditions. After conducting three short demonstration flights with a passenger on board on each occasion, the pilot arranged with the owner to carry out a solo flight to gain thermalling experience. Witnesses reported that the ground roll appeared to be normal, but the aircraft commenced a turn at about 10 feet above the ground while climbing at an unusually steep angle. After the aircraft had reached a height of about 100 feet the climb angle was seen to steepen further, and shortly afterwards the left wing dropped and the aircraft entered a spin. The rotation ceased after one and a half turns with the aircraft then in a steep nose-down attitude. Recovery from this dive was apparently being made when the aircraft struck the ground. No evidence of any mechanical failure or defect was subsequently found. It was considered probable that the unusually steep climb attitude was the result of either the pilot's wish to demonstrate maximum performance, or the unexpected effects of a thermal. The pitch-up which occurred before the stall and subsequent spin was possibly related to the thermal activity.