I'm new at classical mechanics but the text book says there is the torque in the spinning top which generated only by gravitation. It is hard to explain the situation, I've add the link.
http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/imgmech/toppre.gif
There is no rotation axis between $r$(length between the point and center of mass) and $F$(gravitation), and I don't clearly understand the procession of the torque.
I think the precssion of the spinning top is generated by the friction between the surface of the top(close with the point) and ground.
http://www2.picturepush.com/photo/a/13200825/640/13200825.png