Here's why:
* Evolution: Dolphins evolved from land mammals that lived millions of years ago. As they adapted to life in the ocean, their bodies changed, but they retained the ability to breathe air.
* Breathing: Dolphins have a blowhole on the top of their heads that they use to breathe air. They must surface regularly to take breaths.
* Gills: Gills are organs that allow fish and other aquatic animals to extract oxygen from water. Mammals do not have this adaptation.
Let me know if you have any other questions about dolphins or marine life!