Understanding coming from years of exposure leading to memorizing the whole low hanging words of a language and their meaning.

They don't merely understand the words as a concept, they also remember the word sounds attached to the associated concept (and later have to remember the spelling of those words as well). All the while commiting to memory all kinds of facts about the world, starting from their name and the ABC.

Grammar might come closer to exposure-grasping a generalized concept -- then again nobody said understanding concepts is not hugely important, or is done by memorization alone: just that memorization goes hand in hand, and is hugely important in being effective in being able to use and think with not just the concepts but also the relevant facts related to them).

Sometimes those made up words were way more fun than the correct ones.

I could usually see why my little ones combined them that way.

I remember one where the kid was asked, "do you want some half-and-half?"

"No, I want whole and whole!"