It is natural to ask which configurations of propane are most likely to occur in nature. This depends on the accumulated forces between the atoms. Since the hydrogen atoms carry the same charge, each hydrogen atom repels the others. The repulsive force between two hydrogen atoms is inversely proportional to the square of the distance between them. In the applet, drag the rotation sliders to see the how the total repulsive force depends on the location of the hydrogens.
For a first approximation, one may assume that a configuration with a low total repulsive force among its atoms is more stable than a configuration with a higher total repulsive force. Thus, the configuration with the smallest total repulsive force is the likeliest to occur. We want to find the angles of rotation which minimize this force. Manipulate the sliders until you believe you obtain such a minimum force.