La canción de baño

The Bath Song's final chapter ability is not a mana ability. It uses the stack and can be responded to.

You can choose no targets for The Bath Song's final chapter ability just to add {U}{U}. However, if you do choose targets, and all of those targets are illegal at the time the ability tries to resolve, the ability won't resolve and none of its effects will happen. You won't add {U}{U}.