Here's why:
* Breathing: Whales are mammals and need to breathe air. Their blowholes are their nostrils, located on the top of their head.
* Water Pressure: When a whale dives, the immense water pressure would force water into their lungs if the blowhole was open.
* Muscles: Strong muscles around the blowhole seal it tightly, preventing water from entering.
* Opening and Closing: Whales only open their blowholes when they surface to breathe. This is when you see the iconic "blow" of water vapor as they exhale.
Let me know if you have any other questions!