A great kick off question that sparked a rich conversation. I believe I selected it because students had trouble explaining it on a CPM homework assignment. Another explanation that also explains why a negative multiplied by a positive is negative. For example, -3(6) is equal to 0-3(6) because you can add zero to anything and not change it's value (Identity Property of Addition). Which results in an order of operation problem. 0-3(6). Multiply 3(6) first to get 18. 0 minus 18 is -18.
This problem below demonstrates a students curiosity at the beginning of class and reflecting in the last 5 minutes. (http://www.organizedbinder.com)