Likely a sterile medium means an uncontaminated culture (food free of living organisms) that will grow only the new bacteria introduced for analysis. Source(s): http://www.thefreedictionary.com/sterile
Stonepelt was Bluestar's first mentor and a very well-respected senior warrior of ThunderClan. It is revealed in this book that he took her to the Gathering on her second day as an apprentice because she had caught a squirrel very well for a new apprentice, and the squirrel had been as big as she was.
His looks are very vague, age-wise, and his behaviour is that of a hardened veteran towards the band and that of an occasionally insecure 20-something when alone with Demane. Aging the characters in Warrior Cats can be difficult. Cats are considered kittens until they're 12 months old, but in Warriors cats stop being kits at 6 moons.