As S.N.O.T. enters the battlefield, you may stick it onto another creature named S.N.O.T. on the battlefield. If you do, all those creatures form a single creature.
S.N.O.T.'s power and toughness are each equal to the square of the number of S.N.O.T.s stuck together. (One is a 1/1, two are a 4/4, three are a 9/9, and four are a 16/16.)