The Goblet of Fire

The Goblet of Fire was used in the Triwizard Tournament as a method of choosing Champions to compete. Dumbledore drew an Age Line around the cup to prevent underage students putting their name in, although this wasn't a foolproof method and didn't stop the Goblet choosing Harry Potter - despite the fact he was below the qualifying age of 17, and that only one Champion per school was supposed to be picked. The Weasley twins also learnt that magically ageing themselves would also not work, growing lovely, thick white beards when they attempted to do so.