Final answer:
Mammals have the unique characteristic of hair, which sets them apart from other animals. They also possess lungs and can have four legs, but these traits are not exclusive to mammals.
Step-by-step explanation:
Mammals are characterized by several unique features, one of which is the presence of hair. Hair is a defining characteristic of mammals and provides insulation and protection.
Lungs (keyword 1) are present in mammals as well as in many other animals, so they are not unique to mammals. Having four legs (keyword 2) is also not unique to mammals as there are other animals, such as reptiles, that have four legs.
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