a small, clear orb that turns red if you've forgotten something

marble-sized glass ball glows scarlet if you forget something

a magical glass object the size of a large marble and full of white smoke; when held in a person’s hand, the Remembrall glowed scarlet if he or she had forgotten something; Neville’s grandmother sent him a Remembrall, and Draco’s theft of this Remembrall led to Harry being selected as Seeker for the Gryffindor Quidditch team (SS9); Remembralls were banned from examinations at Hogwarts (OP31)