The best way to explain to a friend the connection between glucose entering the body and energy created by the body is through building models.  Legos would be a great means of building such models because of their different shapes, colors, and ease of manipulation.  I would start by creating a molecule of glucose, denoting the various colors for particular atoms, and then add or remove parts while explaining how its changes via the control of specific enzymes during glycolysis.  Upon creating two molecules of pyruvate, I would explain that there are two separate pathways to which pyruvate can enter, aerobic or anaerobic metabolism.  I would demonstrate anaerobic metabolism first as it is less complex between the two forming only one other molecule, lactate, from pyruvate.  Next I would reconstruct the molecules of pyruvate and demonstrate aerobic metabolism in the citric acid cycle.  As before with glycolysis, I would manipulate the specific molecule with each step in the cycle while explaining how my hands are acting as the associated enzymes.  Once my friend understands how each process functions, I would then finish by explaining how energy is created from the synthesis of ATP in the electron transport chain of the inner mitochondrial membrane.