Visit these videos from BrainPop to review the characteristics of amphibians, reptiles and mammals. Their distinct characteristics are what scientists use to classify organisms into their broad classes.

https://www.brainpop.com/science/diversityoflife/amphibians/
https://www.brainpop.com/science/diversityoflife/reptiles/
https://www.brainpop.com/science/diversityoflife/mammals/