Lithography is a printing process based on the fact that grease and water do not mix. An image is drawn on a flat stone or metal plate using a greasy substance so that ink will adhere to it, while non-image areas are made ink-repellent. The printing surface is kept wet, so that a roller charged with oil-based ink can be rolled over the surface, and ink will only stick to the grease-receptive image area. Paper is then placed against the surface and the plate is run through a press. The plate can then be cleaned and re-drawn for additional layers and colours.