The working of a mouse can best be explained with the help of the first mechanical mouse units. They had a trackball in the running surface, which registered the movements of the hand in three directions: vertical, horizontal, and diagonal. The mouse converted the movements in electrical signals and sent them to the computer. The mouse units often got dirty because the trackballs gather a lot of dust. To solve this problem, the optical mouse was developed, which registers movements with light barriers. These were followed by the first wireless mouse units.