Asked by: Gary Barker, Durham
They can’t. The baby on the cover of Nirvana’s iconic album Nevermind is holding its breath, but it isn’t swimming. The breathing response is automatic and part of the mammalian diving reflex. All mammals will automatically close off their trachea and slow their heart rate when their face is immersed in cold water. But human babies younger than four months aren’t strong enough to make effective swimming movements or keep their head above water.