If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves?

I give an answer to the frequently asked question of the article's title, based on an analogy between the description of gravitational waves in the transverse-traceless gauge and the description of an expanding universe in comoving coordinates. Both use freely falling masses to define the coordinate system. Taking advantage of the insight that has been achieved in cosmology, I show how to understand the operation of an interferometric gravitational wave detector in a way that resolves the apparent paradox.