Motion of a spinning top is quite intriguing. When a spinning top is placed on the floor and its tip held in one position, it starts to precess about a vertical axis as shown.

 

Let us take the mass of top as m, its moment of inertia about spinning axis as I, distance of its centre of mass from pivot point is $l$, and its spinning rate is ${\omega }_s$. The rate of precession, that is angular speed at which the top starts to rotate about vertical is $\Omega$. Generally $\Omega$ is much smaller than $\omega$, so in our present discussion we will assume that $\Omega$ does not contribute to angular momentum $\overrightarrow{L}$ and it arises only due to ${\omega }_s$ alone. So $\overrightarrow{L}=I{\overrightarrow{\omega }}_s$. As the top precess, horizontal component of its angular momentum changes. This change is brought by the torque due to weight of top, about the pivot. If top precess with a steady rate, then
$\frac{d\overrightarrow{L}}{dt}={\overrightarrow{\tau }}_{ext}$
Rate of change of horizontal component of $\overrightarrow{L}$ can be calculate easily, as described below