Here's why:
* Saber-tooth tigers (Smilodon) are extinct mammals belonging to the family Felidae, which includes modern-day lions, tigers, leopards, and cheetahs. They were large, powerful predators.
* Opossums are marsupials, belonging to the order Didelphimorphia. Marsupials are mammals that give birth to relatively undeveloped young, which complete their development in a pouch.
While both animals are mammals, they diverged from a common ancestor millions of years ago. Saber-tooth tigers are more closely related to cats, while opossums are more closely related to kangaroos, koalas, and other marsupials.