Lilia picked up her first English phrase from CBeebies: "I know!" I think Postman Pat keeps saying it. The one thing that made the phrase interesting, though, was that her friend Amine (who is 4.5 years old and trilingual himself) used it a lot.
For days on days, she would walk around while playing and exclaim "I know!" This lasted for roughly two weeks, maybe.
I tried to find out whether she actually understood what she was saying, but to no avail.
She has since stopped doing it, fortunately.