What is a greenhouse gas?

Explanation

**Step-by-Step Solution:**

1. **Definition of Greenhouse Gas**: A greenhouse gas is a type of gas that has the ability to absorb infrared radiation emitted from the Earth's surface.

2. **Function of Greenhouse Gases**: These gases trap heat energy in the atmosphere, which helps to maintain the Earth's temperature. This process is known as the greenhouse effect.

3. **Examples of Greenhouse Gases**: Common examples of greenhouse gases include carbon dioxide (CO2), methane (CH4), and water vapor (H2O).

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