One way to disprove is to give a counterexample. One simple condition where the statement becomes invalid is sufficient enough.

So for this case: The product of two irrational numbers is irrational.

So, first we define whta is a rational number. In simple terms, a rational number is a number that can be expressed as a/b. Thus, an irrational number is  anumber which cannot be expressed as such.

So, √2 is irrational, right? Because √2 ≈1.414

√2 x √2 = 2 which is rational.