This is a Remembrall. It turns red when the user has forgotten something.
Warner Bros. / Via acciomigs.tumblr.com
Its most famous user is Neville Longbottom, who can never remember what he's actually forgotten.
Warner Bros. / Via hellogiggles.com
Which is a major flaw in the device, admittedly.
But in fact, it's really obvious – he forgot his robes!
Warner Bros.
Everyone else at the meal is wearing robes, except Neville.
And it's probably a good thing he eventually remembers them, since they end up saving his life a little while later.
Warner Bros.
