____ is used as a rocket fuel.

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for a substance that is used as a rocket fuel.

2. **Recall Common Rocket Fuels**: Think about the types of fuels that are commonly used in rockets. Some examples include liquid hydrogen, kerosene, and liquid oxygen.

3. **Focus on Liquid Hydrogen**: Among the common rocket fuels, liquid hydrogen is widely recognized for its use in space missions. It is known for its efficiency and high energy output.

4. **Consider the Properties of Liquid Hydrogen**: Liquid hydrogen has several advantages:
- It has a low molecular weight.
- It provides a high energy output, which is crucial for launching rockets.
- It is a low-density liquid fuel, making it suitable for use in rockets where weight is a critical factor.

5. **Conclude the Answer**: Based on the properties and common knowledge, the answer to the question is "liquid hydrogen."

**Final Answer**: Liquid hydrogen is used as a rocket fuel.

---