I am having trouble explaining the difference between internal and external forces, and in fact, even *defining* internal forces. Does anyone understand the difference very well? My current example is: Hold an elastic band loosely between both your index fingers. Now, move your fingers apart. This pull is an external force acting on the elastic. However, the elastic has changed. It's now tight and thinner than it was before. The tension of the elastic is an internal force. This is the *result* of the external force of the pull of your fingers. Is this about right?