Here's why:
* Lizards are reptiles, and share many characteristics with other reptiles like snakes, turtles, and crocodiles.
* Cats are mammals, and have completely different body structures, internal organs, and biological functions than reptiles.
* Birds are birds, another distinct class of animals with wings and feathers.
* Tortoises are reptiles, but their shell and lifestyle are significantly different from a lizard's.
* Millipedes are arthropods, a group of animals with an exoskeleton and segmented bodies, entirely different from reptiles.
Therefore, a lizard is the closest comparison to another lizard, as they share the same basic reptilian features.