Tricky...?

 Greg Ross at Futility Closet writes in a very succinct explanatory style that usually makes his puzzles both clever and understandable. I think he came close to being too succinct on this one though that he just put up ("Ice Work"). It looks (and is really) a very simple problem, but the bare bones presentation required several minutes for me to grasp the simple solution. Do others find it initially confusing, or is it more obvious to most readers?:

http://www.futilitycloset.com/2015/01/15/ice-work/