Remove the Knock-Out Flange
Note: This toilet was completely assembled before installation. If installing a toilet that isn't completely assembled, follow the manufacturer's instructions for assembly.
The plumber who roughed in the toilet connection installed a knock-out flange to seal off the waste drain. This keeps sewer gases from entering the home, and prevents debris from falling down the drain. To remove a knock-out flange, tap the center a few times with a hammer, then use locking pliers to remove the center piece.