Are you talking about the product of two negative numbers? There's several ways you can go about internalizing the concept you speak of. One way is to just let them do it on a calculator. Eventually they'll observe that the calculator always returns positive answers. Another way is, if they understand that the product of a positive and a negative number is negative, such as 3(-2)= -6, then explain that making it (-3)(-2), thus introducing the second '-' sign, makes it the opposite and the opposite of -6 is 6.
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