Here's why:

* Hydrogen Fusion: Stars spend the majority of their lives fusing hydrogen into helium in their core. This is the main energy source for stars like our Sun.

* Helium Accumulation: As hydrogen is consumed, helium builds up in the core.

* Hydrogen Shell Burning: Eventually, the core becomes mostly helium. The star then starts fusing hydrogen into helium in a shell surrounding the core. This causes the star to expand and cool, becoming a red giant.

* Helium Fusion: Once the core is hot enough (around 100 million Kelvin), helium can start fusing into carbon and oxygen. This process is called the triple-alpha process.

So, red giants are stars that are in the process of, or have already started, burning helium in their cores. This process marks a significant stage in a star's life cycle.