After a brief survey on the main issues studied in literature about the development of proportional thinking, we present the results of a experimental study on proportionality focused on classroom discussions of three open problematic situations in different contexts. We concentrate on the report of pupils’reasonings which testify their different points of view. We stress the value of the ‘sharing of ideas’ for bringing pupils to understand -through argumentations of their classmates- the weakness of strategies such as the additive one and, more in general, for promoting thinking authonomy and critical scrutiny of own thoughts.