Here is my stab with a rusty spoon (i.e. a very bad analogy based on the link you provided) Particles are like boats on a flat lake, the 2D surface of the lake representing our 3D universe. Without an anchor (massless objects) they go racing about the lake, as the current directs them. If they have mass then they have an anchor hanging down into the lake. This will not simply hang down, as the water is not thick enough, but will swing back and forth like a pendulum. This will cause the boat to oscillate and that will cause ripples. (Just about everything in the analogy is wrong, I'm afraid, but I was trying to emphasize the role the anchor plays and how something can be stationary and still cause ripples.)