Neville Longbottom received a Remembrall in Harry Potter and the Sorcerer's Stone, but why did the inside of the gift turn red? J.K. Rowling's popular fantasy novels became even more popular in the 2000s once Warner Bros. began adapting them into movies. The first film out the gate gave audiences around the world an introduction to Harry Potter and the Wizarding World his story takes place in. In doing so, Harry Potter and the Sorcerer's Stone (as it is known in the U.S.) had to set up various magical elements and devices.
In the midst of explaining pieces of Harry Potter lore that would be important for the franchise's future, the movie also introduced some items that never were fully explored. One example of this is the magical device known as a Remembrall. The large glass ball fills with red smoke whenever the owner forgets something. This red smoke remains inside the Remembrall until the forgotten thing is remembered again.
Harry Potter and the Sorcerer's Stone shows that Neville Longbottom receives a Remembrall as a gift from his Gran. When he reveals the gift to his fellow young wizards, though, it immediately turns red. The movie doesn't explore the importance of the Remembrall any further. It doesn't explain what Neville has forgotten at this moment that the device is trying to remind him of, which makes it a small moment that fans have continued to theorize about. Taking a close look at the Remembrall scene in Harry Potter and the Sorcerer's Stone, though, there is evidence that suggests why Neville's Remembrall turned red.
The leading theory for why Neville's Remembrall turns red is that he forgot his Hogwarts robe. Neville receives the Remembrall while most of the school is gathered inside the Great Hall. He says that he can't remember what he's forgotten once the red smoke fills the gift, but many believe that it is the customary Hogwarts robes. Neville is the only student not wearing the robe during the scene, so the Remembrall could be trying to remind him to put it on.
There are some who don't support this theory because Neville is shown wearing the robe in the very next scene. It doesn't seem like he would have any time to go back to his room to get the robe if he forgot it there. However, if Neville already had the robe with him, then the Remembrall could want him to remember that he needs to wear it. And considering the robe then slowed Neville's fall during his wild first flying lesson in Harry Potter and the Sorcerer's Stone, the Remembrall could've saved his life if that is what brought him to put the robe back on.