In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.

Article: Imagine when you are sailing an ocean for the first time, you see a big fish swimming near your ship. You might be scared, and worried about whether it will turn over your ship for a meal. In fact, it is not a fish at all, but a gentle giant whale. Whales are not fishes but mammals . Their body temperature stays the same, and does not change with the temperature of their environment. They breathe air, so they have to come out of the water to get oxygen. Thus, they cannot afford to fall into a sleeping state for too long, since they need to be awake in order to breathe. It is thought that only half of their brains sleep at a time, so that whales are never completely asleep. Females give birth to the young. They have hair. Although they are not covered in hair or fur like many mammals, whales have some bristles , usually on their heads. Whales are highly social animals with complex languages and intelligence. They communicate with each other using romantic sounds, called "whale songs". Being so large and powerful, these sounds are also loud. Whales are endangered as a result of whaling from the eleventh century to the twentieth. It has attracted the attention of environmentalists and some measures have been taken to protect whales.
Answer: Four.
How many aspects show that whales are mammals?