Introduce and discuss that all motion is relative, ,and must include a reference frame. Determine that objects may have different velocities relative to different reference frames. Indicate that the speed of light postulate states that the speed does not change. Discuss that motion through space is related to motion in time. Einstein postulated that both space and time only exists within the universe. Space and time are the parts of the space-time continuum. If you stand still, you move through time, if you move a little you move through some space but mostly time. If you move at the speed of light, all travel is through space only. Introduce the time dilation as a "stretch of time".