Mosaics are made by setting coloured pieces into "mortar" (cement) which sets hard and holds the pieces in place. Some mosaics are made of round pebbles, and have only two or three colours. Other mosaics are made of marble. Many mosaics, particularly in Italy, are made of terracotta tiles. (Terracotta is "fired" clay that has been baked in an oven.) Terracotta tiles come in many colours and can be used for colourful pictures as well as patterns. 