system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why can babies have water in formula but not actual water before 6 months?
gpt
Think of a person as being made out of chemicals.

A baby is a *very small* pile of chemicals.

Basically, you don't want to dilute the baby too much.

Breast-milk/formula contains relevant chemicals so that a baby stays roughly stable, but pure water will dilute them.

(More details would include stuff like electrolytes and stomach size and osmosis and so on.)