Double black

Lesson of the day: Something may not as easy as we thought when we do it ourselves, but it is worth the effort to do it. Last week Saturday, my company was having a team building activity, obstacle course. The course is coded into 4 difficulties level on different sites, and there are 4 sites. Green, blue, red and black are the colors to represent the easiest to the most difficult one. So, why is it exciting?