When they get to the stage of being able to hold objects, this is a major event. When they get the hang of putting stuff in their own mouths during teething, rather than screaming until you let them bite your finger, is another, similar world-changing event.

This was a mystery to me too, before. Now I understand, and am even mildly interested in other peoples' babies (since I have one to compare them with.)

Parenting also has another similarity with D&D: your adversary is surprisingly intelligent. (when they're grouchy, they'll see through your hacks very quickly and scream until you do it properly, however tired you are. A "hack" in this instance might be giving them the same toy to chew again, rather than a new one from the mat.)