During cooking, sometimes the outer surface of the bottom of the vessel becomes black. This means that

Question

Accepted Answer

The correct answer is: **3. The fuel is not burning completely.**

### Explanation:

When cooking, if the outer surface of the bottom of the vessel becomes black, it indicates that there is incomplete combustion of the fuel being used. This incomplete combustion can lead to the formation of soot, which is a black residue that accumulates on the surface of the vessel.

#### Detailed Analysis of the Options:

1. **The food is overcooked**: 
   - While overcooking can lead to burnt food, it does not directly cause the outer surface of the vessel to become black. The blackening of the vessel is more related to the combustion process rather than the cooking time or temperature of the food itself.

2. **The vessel is made of steel**: 
   - The material of the vessel (whether it is steel, aluminum, or any other material) does not inherently cause the bottom to become black. The blackening is a result of the combustion process and not the type of material used for the vessel.

3. **The fuel is not burning completely**: 
   - This is the correct answer. Incomplete combustion occurs when there is insufficient oxygen for the fuel to burn completely, leading to the production of soot and other byproducts. This soot can accumulate on the bottom of the vessel, causing it to appear blackened.

4. **The fuel used is non-conventional**: 
   - While non-conventional fuels may have different combustion properties, the blackening of the vessel is not solely dependent on whether the fuel is conventional or non-conventional. The key issue is the completeness of the combustion process, which can occur with any type of fuel if there is inadequate oxygen.

### Conclusion:
The blackening of the outer surface of the bottom of the vessel during cooking is primarily a sign of incomplete combustion of the fuel being used. Therefore, option 3 is the most accurate explanation for this phenomenon.