To understand disordered physiology, it is first necessary to determine what constitutes normal function. Liquid sip size during swallowing in healthy individuals has been investigated with varied results. Bolus size is a variable that is manipulated in both research studies and clinical swallowing assessments, so defining normal sip size has relevance in both domains. This study looked at sip size under instruction in experimental tasks and compared it to sip size in free drinking while participants were unaware that drinking was being observed. A statistically significant difference was found in water sip volume between natural drinking (mean = 16 ml) and instructed experimental drinking tasks (mean = 6.6-6.8 ml). This difference far exceeded the magnitude of sip-size variation observed between instructed drinking tasks using different stimuli and as a function of participant's gender or age group.