By putting the clear limits between two eras, which are usually indicated by some important events or some changes that totally changed the society and mankind.
Explanation:
To easily examine historical eras it is important to put clear limits between eras. For example prehistory ended and history when the first states were created and the written letter appeared. This was a great change, that totally changed the human kind. It was something new, totally different, that affected everyone. That is why this two period are indicated like that. Period without states and written letters (prehistory) and the period that had these (history).
