Instead of mere rods and tubes of metal, Hal now saw pistons and cylinders as power-containers. To help his understanding, he visualized how a pinch of gunpowder can easily be put into a gun cartridge. But when the powder is exploded it expands into gases that would fill a house. It is the expansion that shoots the bullet. So it was that the air-gas mixture exploded in a cylinder rushed out to force the piston into unbelievable speed. This speed harnessed to gear and camshaft was the power that was hurling the motor world forward--first on wheels, now on wings.