if an animal lives a certain environment then that animal ususally requires that kind of environment & polar bears live in cold environments ->  a polar bear requires a cold environment  a polar bear requires a cold environment & zoo exhibits should contain all things required by the animals living in those exhibits ->  a polar bear exhibit should contain a cold environment  a body of water is a kind of environment & a pool is a kind of body of water ->  a pool is a kind of environment  a pool is a kind of environment & cold means low in temperature ->  a cold pool is a kind of cold environment  a polar bear exhibit should contain a cold environment & a cold pool is a kind of cold environment -> a polar bear exhibit should contain a cold pool