An old cycle frame was left in open for a few days. A brown layer slowly got deposited on its surface and could not be removed when rubbed with sand paper. What happened actually?

Explanation

### Step-by-Step Solution:

1. **Identify the Material**: The old cycle frame is made of iron (Fe).

2. **Exposure to Elements**: The cycle frame was left open to the atmosphere for a few days. This means it was exposed to oxygen (O2) in the air and moisture (H2O).

3. **Chemical Reaction**: When iron is exposed to oxygen and moisture, it undergoes a chemical reaction to form iron oxide. The general reaction can be represented as:
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