Ultrasound video clips of large urinary bladder calculus:

The large bladder stone is seen rolling from the right lateral wall of the urinary bladder (the patient was lying in right lateral decubitus position) to the posterior surface of the bladder as the patient turns to the supine position.

  I repeated this with the patient lying supine initially. The bladder calculus is seen lying on the posterior wall, and as the patient rotates to the right lateral decubitus position, the urinary bladder stone is seen rolling to the surface of the right wall of the bladder. This simple maneuver helps confirm that the echogenic lesion in the urinary bladder is indeed a calculus, besides helping to visualize the full extent of the urinary bladder stone. Often, one should be able to see if this is a single calculus or multiple, with this simple procedure.

See: http://www.ultrasound-images.com/urinary-bladder.htm for more on this topic.