Here's why:

* Photosynthesis: Plants make food through photosynthesis, a process that uses sunlight, water, and carbon dioxide. This process typically takes place in leaves.

* Adaptation: Cacti live in arid environments where water is scarce. To conserve water, they have evolved to have reduced leaves and instead use their stems for photosynthesis. The stems are green, containing chlorophyll, which is essential for photosynthesis.

Let me know if you have any other questions!