Explanation:

- We begin with an initial mixture containing 470 liters, composed of 5% water.

- This results in the initial water content as 5% of 470 liters: $0.05\times 470=23.5$ liters.

- After the mischievous act, the water percentage increases to 6%.

- Let the amount of water added be $x$ liters.

- The new total water content becomes $23.5+x$ liters.

- The new mixture total remains 470 + $x$ liters.

- The equation for the new water percentage is $\frac{23.5+x}{470+x}=0.06$.

- Solving this, $23.5+x=0.06\times (470+x)$.

- Simplifying gives $23.5+x=28.2+0.06x$.

- Solving for $x$ leads to $x=5$ liters.

So, Option 3: 5 liters is the correct choice.