Why didn't Harry wait around for Dudley to figure out what he had said about the toilet?

From the text:

"They stuff people's heads down the toilet the first day at Stonewall," he told Harry. "Want to come upstairs and practice?"

"No, thanks," said Harry. "The poor toilet's never had anything as horrible as your head down it -- it might be sick." Then he ran, before Dudley could work out what he'd said.