To start, lets say I took a bowling pin and passed it through a two-dimensional universe. An observer in the two-dimensional universe would see a two dimensional slice of the bowling pin expanding and contracting as the bowling pin passed through their universe. Similarly, a 4-dimensional object passing through a 3-dimensional universe would appear to be an 3 dimensional object changing size and shape. When a balloon is inflated, it changes size. Could the inflated balloon and the deflated balloon be different cross-sections of the same four dimensional object passing through out three-dimensional universe? Could all motion simply be changing cross sections of four dimensional objects passing through our universe. Think of the two-dimensional universe again. If you took a 'V' and passed it through the two-dimensional universe, the observer would see a single object appear and then two objects break apart and move away from each other.