On hard, flat ground, like paved roads, hard wheels are more energy efficient than other ways of moving around. However, wheels are not as efficient (they take more energy to move) on soft surfaces, like soils, because they have to deal with "rolling resistance". In rolling resistance, a vehicle loses energy as the shape of the wheel and the surface are deformed (changed), and the surface pushes back against the wheel. Smaller wheels have to deal with more resistance for their size than larger wheels. Softer surfaces deform more than hard surfaces, and they don't come back to their starting shape as well. This causes greater resistance. Compared to concrete, the resisting force on medium-hard soil can be five to eight times greater, and on sand it can be ten to fifteen times greater. While wheels must deform the surface along their entire path, limbs only cause a small deformation at the point that the foot touches the ground.