Almost like werewolves and polymorphs, but instead of having the pain of bone and muscle shifting around, the humanoid is one glop of cells. Each cell, however, can "snuggle up" close, allowing the shapeshifter to have a form that can walk, talk, absorb bullets, etc. The shapeshifter has power over which parts have a particular density. Movie shapeshifters include the T-1000 from Terminator 2, and Kirk's "helper" in Star Trek 6.