In 1991, Dudley Dursley noted to Harry Potter that they stuff people's heads in toilets on their first day at Stonewall High, the Muggle high school that Harry was slated to attend. He offered to help Harry "practise" by stuffing Harry's head in the upstairs toilet, but Harry turned Dudley's words, "Want to come upstairs and practise?" on him by replying "No, thanks. The poor toilet's never had anything as horrible as your head down it -- it might be sick." He then ran away before the slow-witted Dudley could work out his wordplay.
Harry Potter and the Philosopher's Stone (First mentioned)