The water bottle experiment

A class in Da Nang tried a small experiment for three weeks. Every student brought a one-litre bottle to school and filled it in the morning. The rule was simple: finish it before you go home.

Nobody expected much. What they found surprised them. Headaches in the afternoon, which several students had treated as normal, became rare. Two students who often felt dizzy during PE stopped feeling dizzy. The class also noticed something nobody had planned for: people went to the tap less often, because they already had water, and lessons were interrupted less.

The science is not complicated. The body loses water through breathing and sweating all day, and a loss of only two per cent of body water is enough to slow attention and bring on a headache. Thirst arrives late, after the loss has already begun, so waiting until you feel thirsty means waiting too long.

There were limits to what the class proved. Three weeks is short, there was no comparison group, and students who expect to feel better often do feel better. Their teacher pointed this out, and the class agreed it was a fair objection.

Still, the habit stayed. A year later most of the class still brings a bottle. Not because the experiment was perfect, but because carrying water costs nothing and the alternative is a headache at two o'clock.