Immersion rods for heating liquids are made up of metallic substances.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Purpose of Immersion Rods:**
- Immersion rods are used to heat liquids. This means they need to effectively transfer heat to the liquid they are immersed in.

2. **Identifying the Material Requirement:**
- For effective heating, the material of the immersion rod must allow heat and electricity to pass through it easily.

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