What can sound not travel through ?

Explanation

### Step-by-Step Solution:

1. **Understanding Sound Waves**:
- Sound waves are classified as mechanical waves. This means they require a medium (solid, liquid, or gas) to propagate.

2. **Defining Medium**:
- A medium is a substance through which sound travels. In solids, liquids, and gases, sound travels by the vibration of molecules.

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