That's a question I hear fairly often, at least from the somewhat more botanically advanced people who join me in the field. To most, the leafy monocot in the photograph below probably looks like any other "grass," but in fact, it is a sedge, a member of the family Cyperaceae, an evolutionarily advanced family of plants that does not waste energy creating showy petals to attract pollinators. The genus Carex is a member of this family characterized by having perigynia (modified bracts around the flowers and fruits) that completely enclose the flowers except for a pore at the tip from which the style protrudes, and that usually has leaves along the stems (not just at the base of the plant). There are approximately 150 Carex here in the Chicago Region, but the one below is one of the easiest to identify.
You can tell that the sedge above is rhizomatous, meaning that it spreads by horizontal underground stems, leading to the presence of a large colony (as opposed to being cespitose, or clump forming). The coarse leaves are also long and arching, and they are green to yellow-green in color. Some sedges have pubescent leaves and/or sheaths, but this one does not. Many people would consider this one of the lake sedges.
Because this plant spreads so readily by underground stems, it often does not need to produce seed, and therefore does not flower. If you are lucky enough to find the flowers, shown above, you will notice that the perigynia are more than 5 mm long and pubescent, and that the female and male flowers are in separate elongated spikes. In the photograph above, the female, or pistillate, flowers are the yellowish structures, whereas the male, or staminate, flowers are the pinkish-brown structures at the top of the photo.
If these characteristics weren't enough, or if you couldn't find flowers, all you would need to do to correctly identify this sedge would be to check out the ventral sides of the leaf sheaths. As shown above, there is a dark red-purple v- or y-shaped area at the top of the inner band of the sheath. All of these characters point to this sedge as being Hairy-fruited Lake Sedge, Carex trichocarpa. Hairy-fruited Lake Sedge can be found in calcareous meadows, wet prairies, and marshes throughout much of the northern half of the eastern United States, as well as in the adjacent Canadian provinces.