Here's why:

* Photosynthesis: Plants and other photosynthetic organisms use sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen. This process, called photosynthesis, is the foundation of most food chains.

* Food Chains: Animals obtain their energy by consuming plants or other animals that have eaten plants. This energy originally came from the sun.

* Chemical Energy: The glucose produced in photosynthesis stores chemical energy, which is then used by living organisms for growth, movement, and other life processes.

In summary: The sun provides the initial energy that fuels life on Earth through the process of photosynthesis, which ultimately sustains all living organisms.