 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 . 
