I'm not sure. They said that if you soaked certain items in the bath, something would happen. The water would seem to change color, for example.

[There had definitely been a logical reason for bringing the duck. But that doesn't matter, because suddenly, Caesar has an awesome idea. And even if his plan fails, that's okay, because he's sure the attempt will be worth it. So in that brief pause after he finishes that sentence, he picks up the duck and... throws it in his brother's direction, of course.]