When we look at a number next to the entrance of a house it is clear that this number is part of the postal adress of this building. Everyone knows it. It is part of an universal knowledge all humans share, because it is used everywhere in the world. But getting there was not that easy. 2000 years ago the Romans ruled an empire that included parts of Africa and Asia without real characters for numbers. Instead they used letters. You can see them from 1 to 12 on the mechanical clock and also on the sundial of the clock tower. From 1 to 12 is easy to recognize, but imagine the year 1734, when this tower was built: MDCCXXXIV. Confusing, isn't it? Now imagine calculations with these number-letters. Difficult and quite limited. Fortunately a clock needs only 12. But why 12? Why do we divide a day in two parts of 12 hours each? This is a heritage of the Sumerians who lived 5000 years ago in Mesopotamia (today Iraq and Syria)and invented writing: first pictographs, then cuneiform writing. They observed sun, moon, stars and noticed that the circle of the year consists of twelve months. Dividing time by 12 seemed to be given by nature and therefore they divided days in 12 hours daytime and 12 hours nighttime. With the cuneiform writing the Sumerians also invented a system to express numbers for calculating, but it took thousands of years to develop real numbers, invent the "0" and transport the idea of a number that represents "nothing" from India to Arabia (7th century). It took again 500 years for the numbers to become known in Spain and Italy and then spread over Europe. Today our modern life is unimaginable without numbers. They are used for much more than calculating, like postal codes or geographical positioning.