After concluding that the interaction is significant, the difference between the different water means of factor B at a given level of factor A is examined, and we refer to the significant result obtained from the test as the main effect of factor B at that level of factor A.
Within-treatment variation as well as degrees of freedom will still be used to include the results of all treatments in the two-way ANOVA, rather than being recalculated based on the selected treatment group.