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.
Weaning should be a process, rather than an event. You actually begin weaning your baby the very first time you offer him any food other than your milk. (See Starting Solids). Depending on how you go about it, weaning can be abrupt or gradual, and may take days, weeks, or months.