Just to clarify for anyone who doesn't follow that in the context of backwoodsman's post:
If your ball is lying on a movable obstruction, you remove the obstruction and drop a ball.
If your ball is lying other than on an obstruction and is moved when you remove an obstruction you must replace your ball.
And, as a reminder, when dropping you may use a different ball but when replacing you must use the original ball (for exceptions, see Rule 14.2a}