Fudge Flies are a type of wizarding sweet designed to look exactly like small, black, shrivelled flies. They are sold as a novelty confectionery item.
The primary magical property of Fudge Flies is that they audibly buzz when placed in a person's mouth, mimicking the sound and sensation of a live fly. This enchantment is purely for amusement. Their use is straightforward as a treat to be eaten. Ron Weasley used them as an example of how sweets in the Wizarding World are more exciting than their “boring” Muggle equivalents, which lack such interactive magical properties.
Fudge Flies are mentioned exclusively in Harry Potter and the Prisoner of Azkaban. During their third year at Hogwarts, Ron Weasley brought back a bag of Fudge Flies for Harry Potter from the sweetshop Honeydukes after the first weekend trip to Hogsmeade, which Harry was unable to attend. Harry initially mistook the sweets for “little black peppers.” When he ate one at Ron's prompting, he was startled to find it buzzing in his mouth. The moment serves as a piece of world-building, showcasing the whimsical and surprising nature of everyday magic in the wizarding community.