Chemical reactions and catalysts. A chemical reaction involves a chemical change, which happens when two or more particles (which can be molecules, atoms or ions) interact. For example, when iron and oxygen react, they change to a new substance, iron oxide (rust).
It's important for parents to start weaning babies from bottles around the end of the first year and start getting them comfortable drinking from cups. The longer parents wait to start the transition, the more attached kids become to their bottles and the more difficult it can be to break the bottle habit.
Anyway, we usually wean at two months of age. Some time back, by picking up a pencil and paper, sticking my tongue part way out of the corner of my mouth and holding it there; I was able to financially figure out that it was cheaper to take a kid off at that age.