  This article discusses how re-emission of absorbed solar light by centimeter-
to decimeter-sized structures on the surface of an asteroid can create a
component of the recoil force parallel to the surface. Under certain conditions
the west side of stones appear to be on average slightly warmer than their east
sides, thus experiencing a stronger recoil force and increasing the rotation
rate of the asteroid. We study this effect, called the Tangential YORP effect,
in a toy model, replacing stones with walls and simulating heat conductivity in
them. We discuss general trends of the effect, estimate its magnitude, and find
it to be comparable to the normal YORP effect determined by gross-scale
asymmetry of the asteroid. The existence of this effect would modify the
predictions of the YORP acceleration of asteroids.
