Wait 30 minutes after eating before you swim
The old claim turned out to be wrong and was rejected by evidence.
TaughtSwimming on a full stomach causes cramps that will make you drown. Wait at least half an hour. Preferably an hour.
NowThere is no evidence for it. Digestion diverts some blood to the gut, but nowhere near enough to cause dangerous cramping, and drowning statistics show no association with eating.
What actually happened
The physiological story sounds plausible, which is why it survived: blood goes to the stomach after a meal, so the muscles are shortchanged and cramp. In reality the body has plenty of circulatory capacity to do both, which is why you can go for a walk after lunch without collapsing.
The American Red Cross's own scientific advisory review found no evidence supporting a mandatory waiting period, and Duke University researchers examining drowning data found the pattern absent. Mild cramping after a very large meal during hard exertion is possible and unpleasant; drowning because you ate a sandwich is not a documented phenomenon. The genuine drowning risk factors are alcohol, not being able to swim, and no supervision. A generation of children sat miserably beside pools watching a clock for nothing.
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Who was taught this
Still standard through 2011, so anyone who finished school between 1950 and 2005 learned the earlier version.