Dumbledore's Army
In "Harry Potter and the Order of the Phoenix", Hermione Granger creates fake, enchanted Galleons (Wizard money) that are used for communication between members of Dumbledore's Army (the DA). Like real Galleons, the coins have numerals around the edge. On normal Galleons these serial numbers signify the goblin who cast the coin; on the enchanted Galleons, the numbers represent the time and date of the next D.A. meeting, and automatically change to match whatever numbers Harry Potter sets on his coin. Due to the coins being infused with a Protean Charm, once Harry Potter alters his, every coin changes to suit. The coins grow hot when the numbers change to alert the members to look at their coins.