Once devices have been distributed around us, it is essential to model the context in which we interact on a daily basis in an appropriate way. In this sense, we should ensure that the devices chosen should be completely suited to needs. Our aim is to create applications to facilitate the user's daily activities, without any extra effort in terms of interaction, if possible. In this work we present an approach to the modeling of contexts through RFID technology, identifying users as implicit inputs to the system and offering services as implicit outputs. The particular contexts studied are the classroom and the conference site.