Facing mathematics classrooms in primary schools, one may get the impression that the claims for differentiation in order to cope with heterogeneity are fulfilled in an exhaustive way. One can find numerous supporting manipulatives, learning games, several worksheets etc. According to ›open learning situations‹ (a partly dangerous phrase), forms of organisation became open in the sense that children are allowed to deal with different contents, at different places and different times as well as in different kinds. But: Specific criteria have to be assured to prevent that this practice goes down to make-work.