………….. Is used as a rocket fuel.

Explanation

**Step-by-Step Solution:**

1. **Understanding Rocket Fuel**: Rocket fuel is a substance that produces thrust when burned. It needs to be highly combustible and produce a significant amount of energy.

2. **Identifying the Fuel**: The video mentions that the fuel used in rockets is liquid hydrogen. This is because it burns quickly and efficiently.

3. **Properties of Liquid Hydrogen**: Liquid hydrogen has a high calorific value, which means it produces a lot of energy when burned. This is crucial for rockets that need to generate enough thrust to escape Earth's gravity.

4. **Combustion Process**: In the rocket engine, liquid hydrogen reacts with oxygen from the air (or sometimes stored oxygen) to produce combustion, resulting in a large amount of energy and thrust.

5. **Conclusion**: Therefore, the answer to the question is that **liquid hydrogen** is used as a rocket fuel.

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

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