Air is the fuel. A paraglider has no propeller, yet it can remain aloft for hours when its pilot finds a thermal, a column of sun-warmed air whose buoyancy drives it upward. That is the bargain. Uneven ground heating starts it, while cooler air sinks nearby and the rising parcel may continue climbing until it mixes with surrounding air or spreads beneath a cumulus cloud.Skill, not magic, decides r...More