In the animated American TV show "The Flintstones", what does Fred Flintstone use to power his Stone Age car?

"The Flintstones" is an American animated sitcom produced by Hanna-Barbera Productions. The series takes place in a romanticized Stone Age setting and follows the activities of the titular family, the Flintstones, and their next-door neighbors, the Rubbles. It was originally broadcast on ABC from September 30, 1960, to April 1, 1966, and was the first animated series to hold a prime-time slot on television.

In the sitcom, Fred Flintstone, the main character, drives a Flintmobile, as it is sometimes known. It is powered by something Fred always carries with him — his feet. The vehicle, which is made of stone and wood and must weigh hundreds, if not thousands, of pounds, is somehow easily pushed along by Fred's powerful legs. To stop, Fred simply digs his feet into the ground, grinding the car to a screeching halt in a maneuver that can only be safely performed in animation.