In the Harry Potter universe, a Howler is a magical letter that is enchanted with a message from the sender that projects their voice at high volume. Howlers come in distinctive red envelopes, making them easy to spot in a pile of post. The purpose of a Howler is to deliver a message expressing significant anger or displeasure in a manner which standard writing could not adequately convey. Although they are never gladly received, ignoring them only increases the volume and vitriol of the message, and a Howler will ultimately explode if not opened. The physical temperature of the Howler begins to rapidly increase upon delivery, and it will explode if left unopened for too long. This mechanism ensures that the recipient will open the Howler, even though they know that it contains an unpleasantly loud message. Once the message has been received, the envelope bursts into flames, leaving only ashes.