The way they hear things, and try to put them in a context they understand.
Owen, for example, used to call a bath, "bathing soup." Like this whole amalgamation of bathing suit and getting into water and, well, soup. And also he called a bathing suit a "baby suit."
And then, because they hang out together, and pass words back and forth like colds, Lilly grew up calling a bath "baby soup." Which actually strikes me as surprisingly accurate.
Recently, Lilly got a pair of plastic light up high-heeled shoes. Not from us. That's the kind of horrifying toy that only Aunts would buy children. Anyway, she loves them. But she calls them her, "high skills."
Again, surprisingly accurate.