It uses both positive and negative poles as a mechanism to complete challenges. (teachermagazine.com.au)
The positive and negative poles on magnets will help students understand the relationships: how a positive plus a negative moves to the right, how two negatives make a positive. (thoughtco.com)
As pointed out in Jump starting a dead battery: Connect black wire to negative pole of battery or grounded metal? (mechanics.stackexchange.com)