Why are giraffes and whales mammals?

Giraffes are classed as mammals. They are not related to retiles in any way.

They are classed as mammals because they are vertebrates (They have a backbone, or spine), they are endothermic (this is sometimes called "warm blooded", and basically means they regulate their own boy temperature by using internal mechanisms, like sweating and panting), they have hair on their bodies, and they produce milk to feed their young, as well as giving birth to live young.

Reptiles on the other hand are completely different. They are Ectotherm (this is what's known as "cold blooded" and means they use external means, such as basking the sun to warm up, or hiding in the shade to cool down), they have scales instead of hair, and they lay eggs instead of giving birth to live young, and do not produce milk for them, as hatchlings do not need it.