I have seen many parents giving water for their babies to drink besides the milk intake. It is also a normal advice from our parent and grandparent generation that babies should drink more water. However, after read the following articles then only realized water can hurt babies under 6 months.
The articles clearly stated that babies younger than 6 months old may put at risk for water intoxication and potentially life-threatening condition if consume too much water.
Further information may refer to the links belows:
http://www.msnbc.msn.com/id/24760916/from/ET/
http://www.parenttime.com/babytips/babydrinkingwater.html
http://www.associatedcontent.com/article/175467/should_a_baby_drink_water.html
