Here's why:

* Sulfur (S): The equation shows one sulfur atom on each side (balanced).

* Oxygen (O): The equation shows three oxygen atoms on the left side (SO2 + H2O) and four oxygen atoms on the right side (H2SO4).

* Hydrogen (H): The equation shows two hydrogen atoms on each side (balanced).

To balance the equation, you need to add a coefficient in front of the water molecule:

SO2 + 2H2O → H2SO4

Now the equation is balanced:

* Sulfur (S): One on each side.

* Oxygen (O): Four on each side.

* Hydrogen (H): Four on each side.