If we do not clean our teeth and mouth after eating food, then many harmful micro-organisms called A begin to grow and live on the tooth. These micro-organisms act on the substance B present in the left-over food particles sticking to the tooth to form C. The C eats up the parts D and E of the tooth gradually and ultimately make a hole in the tooth. When this hole reaches part F of the tooth, our tooth becomes painful and we get toothache.
What is F?

Answer

To solve the question step by step, we will identify the components mentioned in the question regarding tooth decay.

### Step-by-Step Solution:

1. **Identify Micro-organisms (A)**: The harmful micro-organisms that grow on teeth when we do not clean them are bacteria. These bacteria thrive on leftover food particles.

**Hint**: Think about what types of micro-organisms are commonly associated with dental hygiene.

2. **Identify Substance (B)**: The substance present in leftover food particles that the bacteria act upon is sugar. Sugars are often found in many foods and are a primary food source for bacteria.

**Hint**: Consider what types of food leave residues that can stick to teeth and provide energy for bacteria.

3. **Identify Product (C)**: When bacteria act on sugar, they produce acid. This acid is harmful and contributes to tooth decay.

**Hint**: Think about the by-products of bacterial metabolism, especially in the context of dental health.

4. **Identify Parts of the Tooth (D and E)**: The parts of the tooth that are gradually eaten away by the acid produced by bacteria are the enamel and dentine. Enamel is the outermost layer, and dentine is the layer beneath it.

**Hint**: Recall the structure of a tooth and what layers are present.

5. **Identify Painful Part (F)**: When the hole created by tooth decay reaches the pulp cavity, it causes pain. The pulp contains nerves and blood vessels, which signal pain to the brain when irritated.

**Hint**: Consider which part of the tooth is sensitive and contains nerves that can feel pain.

### Final Answer:
F is the **pulp** of the tooth.