In fossil fuels, the sun's energy has been converted to -- energy.

Explanation

**Step-by-Step Solution:**

1. **Understanding Fossil Fuels**: Fossil fuels are natural substances formed from the remains of ancient plants and animals. They include coal, oil (petroleum), and natural gas.

2. **Source of Energy**: The primary source of energy for fossil fuels is the sun. Plants use sunlight to perform photosynthesis, converting solar energy into chemical energy stored in their biomass.

3. **Conversion Process**: Over millions of years, the remains of these plants and animals get buried under layers of earth. The heat and pressure from the earth transform this biomass into fossil fuels, effectively converting the sun's energy into chemical energy.

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