Glass does not allow _______________ to flow through it.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Properties of Glass**:
- Glass is classified as an insulator. Insulators are materials that do not conduct electricity because they lack free-moving charged particles.

2. **Identifying Free Electrons**:
- In conductive materials (like metals), free electrons can move easily, allowing electric current to flow. In contrast, insulators like glass do not have these free electrons.

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