A paint brush is not simply a tool for coloring in a shape. The marks you make with it influences the way a viewer interprets what they're looking at. Think about the direction in which you're moving your brush as you paint; it does make a difference.
Both circles in the photo above have only been painted roughly, yet already the one on the right looks more like a sphere than the one on the left. This is the result of the brush marks following the form or contour of a sphere.
Botanical artists call it painting with the "direction of growth". If you find this hard to visualize or decide, touch the object and see which way you instinctively move your hand over it (not the direction your fingers curl).
