The ongoing developments in tissue engineering to build lasting, functional cell assemblies highlight the importance of cell-to-cell interactions in the formation of a multicellular structure. Several techniques have been developed over the last years to build 3D cell aggregates and to investigate their cohesivity. Here we introduce a new method: an external magnetic force drives magnetically labelled cells to a non– specific substrate to form an aggregate of prescribed geometry. We can easily monitor the temporary application of magnetic gradients and their geometry. We measured the dimensions of the so–formed mound after the release of the magnetic force to value its cohesion without any external constraint. We then investigated the organization in the aggregate depending on the number of cells and the applied magnetic force. For particular experimental conditions, the force-induced multicellular aggregate is unstable and disassembles over a short period of time. Studying different cell types (the aggregating amoeba Dictyostelium discoideum and epithelial progenitor cells), we find preliminary evidences that cell-cell interactions are a critical parameter for both the formation of the multicellular structure and its disassembling dynamics.