Severus Snape's Potion Riddle

It really is extremely easy to solve once you think about it.

For those of you who may want to try it out: http://georgewiseman.deviantart.com/art/Snape-s-Potion-Riddle-209343544

Having it visually in front of you helps a lot - of course this may not have been the way JKR imagined it, but hey ho.