As you try to help Swampy by solving the puzzles in Where's My Water?, it's important to know the different elements and their functions:
Swampy loves to shower; what he needs is clean, blue water for himself and his rubber duckies. Water, however, also makes algae grow.
Contaminated Water (purple)
Dirty, contaminated water is bad for Swampy and his rubber duckies. Contaminated water also dissolves algae. When mixed with toxic ooze, an explosive compound is created that can blast away stone.
Toxic Ooze (yellow)
While both forms of water pool in the gaps in the earth, thick, toxic ooze eats right through. Although toxic ooze and contaminated water make a volatile substance, toxic ooze mixed with algae creates stone.
Green algae isn't necessary harmful to Swampy or his rubber duckies, but when mixed with clean water it can grow out of control and block the spouts of the pipes leading to Swampy's showerhead.
Make sure to water all the rubber duckies and find all the collectibles to unlock bonus levels. To view found items and access bonus levels, tap on "Collection" from the level select screen.