In the Harry Potter universe, a boggart is a shape-shifting creature that takes on the form of whatever most frightens the person who encounters it. They are amortal, non-beings that are generated and sustained by human emotions, much like poltergeists and Dementors. Boggarts can be made to disappear, but more will inevitably arise to take their place. When facing a boggart, it is best to have someone else present to cast the "Riddikulus" spell, which forces the boggart to assume a less-threatening and hopefully comical form. If the caster is able to laugh aloud at the boggart, it will disappear at once. Some famous boggarts in the Harry Potter universe include the Old Boggle of Canterbury and the Bludgeoning Boggart of Old London Town.