___________ is used as a solid lubricant for machine parts.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a substance that is used as a solid lubricant for machine parts.

2. **Identifying the Problem**: Machines have moving parts that come into contact with each other, which creates friction. This friction can lead to wear and tear, reducing the efficiency of the machine.

3. **Role of Lubricants**: Lubricants are substances that help reduce friction between moving parts. They can be in liquid or solid form. In this case, we are looking for a solid lubricant.

4. **Finding a Suitable Solid Lubricant**: Among various substances, we need to identify a solid that has slippery properties.

5. **Identifying Graphite**: Graphite is a well-known solid that is slippery. It has a layered structure that allows the layers to slide over each other easily, making it an effective lubricant.

6. **Conclusion**: Therefore, the answer to the question is that **graphite** is used as a solid lubricant for machine parts.

**Final Answer**: Graphite

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