Where Do They Find These Things

I have 2 kids one is a teen and one is a toddler, My teen walks up to little one the other day and says "give me five" little one reaches out to shake her hand, and she says " no give me five" and little one reaches to shake her hand again. Well finally my teen reaches to shake her hand and little one squints her eyes, shakes her hand and says "Tank u, Tank u". We where cracking up and now since she got a reaction from us she is doing it all the time, I have no clue where she seen or heard this.