The bacteria present on uncleaned teeth convert the sugar in food into a substance X which causes tooth decay. Name the substance X.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the name of a substance (X) produced by bacteria on uncleaned teeth that leads to tooth decay.

2. **Identifying the Role of Bacteria**: Bacteria present in the mouth feed on sugars from food particles that remain on uncleaned teeth.

3. **Conversion Process**: These bacteria convert the sugars into an acidic substance. This process is crucial because it highlights the role of bacteria in dental health.

4. **Impact of the Acid**: The acid produced by the bacteria is harmful to the teeth. It leads to the erosion of the enamel, which is the hardest layer of the tooth.

5. **Result of Acid Action**: The action of the acid on the enamel results in tooth decay, characterized by the formation of cavities or holes in the teeth.

6. **Conclusion**: Therefore, the substance X that causes tooth decay is **acid**.

### Final Answer:
The substance X is **acid**.

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