1. Whales are warm-blooded, while fish are cold-blooded. This means that whales can regulate their own body temperature, while fish cannot. Whales have a layer of blubber that helps them to stay warm in cold water.
2. Whales breathe air, while fish breathe water. Whales have lungs, while fish have gills. Whales must come to the surface to breathe, while fish can breathe underwater indefinitely.
3. Whales give birth to live young, while fish lay eggs. Whales are viviparous, which means that they give birth to live young that have been developing inside the mother's body. Fish are oviparous, which means that they lay eggs that hatch into young fish.
4. Whales have mammary glands and produce milk to feed their young, while fish do not. Whales are mammals, which means that they have mammary glands. These glands produce milk, which the mother whale uses to feed her young. Fish do not have mammary glands and do not produce milk.
In addition to these four main reasons, there are a number of other smaller differences between whales and fish. For example, whales have hair, while fish do not. Whales also have a four-chambered heart, while fish have a two-chambered heart.