|MadSci Network: Physics|
Indeed, the diameter of the straw should not have a (big) effect on the height to which water can be drawn upwards. Water will rise slightly in small straws due to capillary action, even if no suction is applied. I do not see how Pascal's Principle (pressure is transmitted equally to all portions of a container holding an incompressible fluid) would explain any difference between narrow and wide straws. I cannot explain the outcome of your student's experiment. I speculate that there might be differences in the materials from which the straws are made, or that he might be able to fit his lips and tongue more firmly against a wide straw than a narrow one. As far as I can tell, "physics" does _not_ explain the result.
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