Here's why:

* Oxidation number represents the hypothetical charge an atom would have if all bonds were 100% ionic.

* In a neutral molecule, the sum of the oxidation numbers of all atoms must equal zero.

* Hydrogen typically has an oxidation number of +1.

* Chlorine typically has an oxidation number of -1.

Let's break down CH2Cl2:

* There are two hydrogen atoms (+1 each) = +2

* There are two chlorine atoms (-1 each) = -2

* Therefore, the carbon atom must have an oxidation number of 0 to balance the molecule.

So, the oxidation number of CH2Cl2 is 0.