Conventional wisdom and indirect studies suggest that the mechanical properties of nanoparticles can be considerably different than their bulk properties would predict. However, little is actually known about their mechanical behaviour because of the practical difficulties in investigating individual particles. Direct experimental studies of these properties require knowledge of the crystallographic orientation, size and microstructure of the nanoparticle in order to be complete. By deforming a single nanoparticle in the transmission electron microscope we have been able to determine each of these parameters of an isolated silicon nanoparticle a priori. With this approach, we could then directly examine dynamic deformation processes and demonstrate the first direct observation of plasticity-induced cleavage fracture of a silicon nanoparticle in compression.