Which word does not fit with the others: Pool, Ocean, Lake, River?

The word "Pool" does not fit with the others because it refers to a smaller, often man-made body of water, typically used for swimming. In contrast, "Ocean," "Lake," and "River" all describe natural bodies of water that are generally much larger and part of a natural water system. Oceans are vast bodies of saltwater, lakes are sizable inland bodies of freshwater (or sometimes saltwater), and rivers are flowing watercourses that can connect lakes and oceans. Hence, the distinct nature and scale of a pool differentiate it from the other three, which are all significant components of natural aquatic environments.