This article is the second post of a series of ten contributions about a better understanding of the different aspects of Open Science. I want to collect material to develop a taxonomy of Open Science (TOS). Here I will outline the rationale and significance behind the Open Citation movement. Citations are the links that knit together our scientific and cultural knowledge. They need to be freely accessible, separated from their sources, such as journals, articles or books, machine-readable, and reusable. They have to be open to facilitate research on their structure and relationships.