Top: H₂O 2₁₂–1₀₁/CO 16–15 line ratio as a function of velocity toward the six Class 0 sources where the H₂O line is observed with HIFI. The brightness of the color indicates the number of observations passing through a given poin – brighter represents a higher number. The channel size is 3 km s^-1. Bottom: same as top but for the H₂O 1₁₀–1₀₁/CO 16–15 line ratio where Class 0 and I sources are separated (red and blue, respectively). The total mean ratio is displayed.