In this video segment adapted from Shedding Light on Science, light is described as made up of packets of energy called photons that move from the source of light in a stream at a very fast speed. The video uses two activities to demonstrate that light travels in straight lines. First, in a game of flashlight tag, light from a flashlight travels directly from one point to another. Next, a beam of light is shone through a series of holes punched in three cards, which are aligned so that the holes are in a straight line. That light travels from the source through the holes and continues on to the next card unless its path is blocked.