One mechanical technique is dropping globules of molten glass into a groove made by two interlocking parallel screws. As the screws rotate, the marble travels along them, gradually being shaped into a sphere as it cools. Colour is added to the main batch glass and/or to additional glass streams that are combined with the main stream in a variety of ways. For example, in the "cat's-eye" style, coloured glass veins are injected into a transparent main stream. Applying more expensive coloured glass to the surface of cheaper transparent or white glass is also a common technique.