AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Tooth decay starts when the pH of the mouth falls below 5.5. At this pH, the tooth enamel (calcium hydroxyapatite) gets corroded by acids produced by bacteria.
The key fact to state is the specific pH value: lower than 5.5. Examiners expect the exact threshold. Mentioning that enamel gets corroded and that bacteria produce the acid adds completeness but the pH value is the core 1-mark answer.