Here's why:
* Both are tetrapods: They both have four limbs (or descended from four-limbed ancestors).
* Both are cold-blooded: They both rely on external sources to regulate their body temperature.
* Both lay eggs: Though amphibians can have larval stages, they reproduce by laying eggs.
Think of it this way:
* Reptiles are like a specific kind of car, like a sedan.
* Amphibians are like a different kind of car, like a truck.
Both are vehicles, but they have different characteristics and functions.
Antonyms are words that have opposite meanings. For example, "hot" and "cold" are antonyms. There is no word that describes the opposite of a reptile or amphibian.