We coined the term "mobile lessons" for lessons held outside traditional classrooms. During these lessons, all actors are mobile and must move to perform the required tasks. Themes tackled in such lessons may be as varied as geography, history, ecology, dialects in linguistics... Mobile lessons are not a new teaching strategy, but mobile devices may render them more efficient and more attractive. The aim is to put students in conditions germane to the ones in which experts work. We implemented in Java a toolkit for creating and using mobile lessons and for monitoring students on the field. Contents and questions are in XML. Using this software, teachers of a high school in Sardinia (Italy) developed and experienced a mobile lesson on the archaeological site of Nora. Under the light of this experiment, a wireless, distributed and more sophisticated version of the software was implemented.