We know from everyday experience how light makes its presence felt. But how can one tell whether or not there's a gravitational wave in a given region of space? The answer: The gravitational wave shows its presence by characteristic, systematic distortions of distances. For an especially simple wave coming through the computer screen towards the reader, these distortions are shown in the animation, left. To make them visible in this setting, the extent of the distortions has been exaggerated greatly. As can be seen, first all the vertical distances contract, while horizontal distances are stretched; after a while, there is the reverse effect of horizontal shrinkage and vertical stretching.