evidence: In general, your baby doesn't need to drink water until they're 6 months old. Until then, they get all the hydration they need from breast milk or formula, even in hot weather. Once your baby is 6 months old, it's okay to give them sips of water when they're thirsty. Don't overdo it, though, or you might give them a tummy ache or make them too full to eat well. And never give your baby water as a replacement for breast milk or formula, which are full of the nutrients babies need. Besides that, your baby's kidneys aren't fully mature yet, and too much plain water could throw off their electrolyte balance. [...] Giving a baby younger than 6 months old too much water can interfere with their body's ability to absorb the nutrients in breast milk or formula. It can also make them feel full, so they'll eat less.