Sunlight is the primary source of energy for most living things on Earth. Plants use sunlight to convert water and carbon dioxide into the sugars they need to grow. This process, called photosynthesis, is the foundation of the food chain. The energy in the sugars produced by plants is passed on to animals when they eat plants. Ultimately, all the energy that fuels life on Earth comes from the sun.
Other related questions:
Q: How is sunlight a primary source of energy?
A: Sunlight is a primary source of energy because it is the source of all life on Earth. Plants use sunlight to photosynthesize, and animals either eat plants or eat other animals that have eaten plants.
Q: Is sun a primary source?
A: Sun is a primary source of light and heat for Earth.