Here's why:
* Vertebrates: Mammals belong to the group called vertebrates. This means they have a backbone or spinal column, which is a series of bones that protects the spinal cord and provides structural support for the body.
* Mammalian Characteristics: Having a backbone is a fundamental characteristic of all mammals, along with other features like hair, mammary glands, and giving birth to live young.
So, if an animal doesn't have a backbone, it cannot be a mammal.