- Sludge has a normal consistency in terms of movement. It behaves the same as Water, Cold Sludge and Hot Sludge. Like Poison Water from Where's My Water? and Purple Water from its own sequel, Sludge is needed to complete all Doofenshmirtz Evil Incorporated levels and to collect Evil Gnomes. If it enters Perry's or an Agent's pipe, the level is failed. Sludge will also kill any other Gnomes.
- Reactions with Fluids
- Notable Reactions with Objects