 Portal 2 integrates a game mechanic from Tag Team's Tag: The Power of Paint: paint-like gels that impart special properties to surfaces or objects they coat. Impressed by Tag, Valve hired its creators to develop the mechanic further and only later decided to include it in Portal 2. Valve's vice-president of marketing said, "the decision to combine their tech with Portal 2 came naturally". Journalists have likened Tag to Narbacular Drop, the DigiPen student project that became Portal. The Repulsion (jumping) and Propulsion (running) gels in Portal 2 originated in Tag. Using the third Tag gel, which allows the character to walk on any coated surface regardless of gravity, gave playtesters motion sickness. It was replaced by Conversion gel, which integrates with the portal mechanic. The gels give the player more control over the environment, which increased the challenge for the puzzle designers. The gels are rendered using fluid dynamics routines specially developed at Valve by the former Tag Team.
