  In modern society people are being exposed to numerous information, with some
of them being frequently repeated or more disruptive than others. In this paper
we use a model of opinion dynamics to study how this news impact the society.
In particular, our study aims to explain how the exposure of the society to
certain events deeply change people's perception of the present and future. The
evolution of opinions which we consider is influenced both by external
information and the pressure of the society. The latter includes imitation,
differentiation, homophily and its opposite, xenophobia. The combination of
these ingredients gives rise to a collective memory effect, which is triggered
by external information. In this paper we focus our attention on how this
memory arises when the order of appearance of external news is random. We will
show which characteristics a piece of news needs to have in order to be
embedded in the society's memory. We will also provide an analytical way to
measure how many information a society can remember when an extensive number of
news items is presented. Finally we will show that, when a certain piece of
news is present in the society's history, even a distorted version of it is
sufficient to trigger the memory of the originally stored information.
