fire extinguisher is

Explanation

### Step-by-Step Text Solution:

1. **Definition of Fire Extinguisher**:
A fire extinguisher is an active fire protection device designed to control or extinguish small fires, typically in emergency situations.

2. **Structure of Fire Extinguisher**:
It usually consists of a handheld cylindrical pressure vessel that contains a fire extinguishing agent.

3. **Function of Fire Extinguisher**:
The primary function of a fire extinguisher is to discharge the agent to extinguish or control a fire.

4. **Common Fire Extinguishing Agents**:
One of the most common agents used in fire extinguishers is carbon dioxide (CO2).

5. **Mechanism of CO2**:
CO2 works by cutting off the fire's contact with oxygen, which is essential for combustion, thereby extinguishing the fire.

6. **Conclusion**:
In summary, a fire extinguisher is a crucial safety device that helps manage small fires effectively, and CO2 is a widely used extinguishing agent due to its ability to remove oxygen from the fire environment.