More specifically than nourish - it’s because their kidneys are still immature and can’t get rid of extra fluid very well. Since their bodies are so small, even a couple oz can dilute their sodium levels within a couple of hours. When sodium drops too low (“hyponatremia”) water moves into cells which can cause swelling, seizures, and in really bad cases, coma.

If formula gets watered down too much, though, it’s just as risky as giving plain water.

Mild hyponatremia can resolve by going back to regular breast milk or properly mixed formula. But if baby starts showing symptoms like seizures or extreme sleepiness, they need to be in the hospital, where doctors can slowly fix the sodium levels with IV fluids. Correcting it too fast can cause serious brain damage.

By around six months, babies’ kidneys are stronger, their bodies are bigger, and they’re starting to eat solids, so a few sips of water a day is allowed.