Duck and "quack" were two of the girls' first words. Now they have moved on. The duck no longer says quack. It says "afflaaaac" (or a variation thereof). This new trick is thanks to an Aflac duck stuffed animal I received at work a few years ago and recently brought home.
The girls will get going with this and it is hilarious. I haven't been able to catch the full effect on video with all three doing it at once, but this gives you the idea: