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

The word "Pool" stands out as it represents a man-made body of water, typically smaller and designed for recreational use, often found in backyards or community facilities. In contrast, the other terms—"Ocean," "Lake," and "River"—all refer to natural bodies of water. Oceans are vast saltwater bodies located between continents, lakes are large inland bodies of standing freshwater, and rivers are flowing bodies of water that usually flow toward oceans or lakes. This fundamental distinction between natural and artificial water bodies is what makes "Pool" the word that does not fit with the others.