Every teenage mystery-solving team needs someone to act as the brains of the operation, and that role is filled by the brilliant, if a little clumsy, Velma.
Velma is the one who finds the clues, puts them all together and usually ends up solving the case, despite Scooby and Shaggy getting in the way. Which just goes to show that behind every cowardly teenage kid and his craven canine is a strong, resourceful, intelligent woman, who loses her glasses almost every episode.