sentences: the grand canyon is a kind of canyon & most canyons are formed by flowing rivers through erosion over long periods of time ->  the grand canyon is formed by flowing rivers through erosion over long periods of time  a river is a kind of flowing body of water & the colorado river is a kind of river next to the grand canyon ->  the colorado river is a flowing river next to the grand canyon  the grand canyon is formed by flowing rivers through erosion over long periods of time & the colorado river is a flowing river next to the grand canyon ->  the grand canyon was formed by the flowing colorado river over long periods of time  the grand canyon was formed by the flowing colorado river over long periods of time & if something is formed by something else, then that something else is required by that something ->  forming the grand canyon requires the flowing colorado river  forming the grand canyon requires the flowing colorado river & if something requires something else then that something else is important to that something ->  the flowing colorado river is important for forming the grand canyon  a river is a kind of flowing body of water & the colorado river is a kind of river ->  the colorado river is a flowing body of water  the flowing colorado river is important for forming the grand canyon & the colorado river is a flowing body of water -> a flowing body of water is important for forming the grand canyon