What concepts must student know and understand for a teacher to explain adequately that the product of two negative number is positive? There are many in the background; 1× a = a, 0×a = 0, a+0 = a, a + ( -a ) =0, a + ( b+ c) =(a + b)+ c , and so forth. However , the goal of this article is not to ask whether high school student can reproduce a complete proof but rather to get a feel for the reasoning that might be appropriate for them, given their knowledge of the structure and consistency present in the number system.