NP has a simple model where the strokes you draw are "wires" not solid shapes. it's the end of the wires that join to other wires and make pivots so if you want an explicit pivot point make sure that the end of one of your strokes is at the pivot. wires strokes joined in this way do not collide with each other.

similarly if you want to make a wheel you can't just join to thin air in the middle of a circle - you must add some spokes. also, since you don't want your axel (or wheel holder) to collide with the wheel when it is joined to the spokes it is best to draw the wheel and spoke(s) as a single stroke.

clear as mud??