There is general consensus that drinking water facilitates certain cognitive processes. However, it is not yet known what mechanism underlies the effect of drinking on performance and these may be different for different cognitive processes. We sought to elucidate the mechanisms involved by establishing at what stage of the drinking process cognitive performance is influenced. We examined the effect of mouth rinsing and mouth drying on subjective thirst and mood, visual attention and short term memory in children. Data are reported from 24 children aged 9- to 10-years. Children’s performance was assessed in three conditions - mouth drying, mouth rinsing and a control (no intervention). In each condition they were assessed twice - at baseline, before intervention, and 20 minutes later at test. Mouth rinsing improved visual attention performance, but not short term memory, mood or subjective thirst. The effects of mouth drying were more equivocal. The selective nature of the results is consistent with suggestions that different domains of cognition are influenced by different mechanisms.