Available macroscopic theories-such as the Johnson-Kendall-Roberts (JKR) model-predict spherical particles to stick to each other at small collision velocities v; above the bouncing velocity, v_{b}, they bounce. We study the details of the bouncing threshold using molecular dynamics simulation for crystalline nanoparticles where atoms interact via the Lennard-Jones potential. We show that the bouncing velocity strongly depends on the nanoparticle orientation during collision; for some orientations, nanoparticles stick at all velocities. The dependence of bouncing on orientation is caused by energy dissipation during dislocation activity. The bouncing velocity decreases with increasing nanoparticle radius in reasonable agreement with JKR theory. For orientations for which bouncing exists, nanoparticles stick again at a higher velocity, the fusion velocity, v_{f}, such that bouncing only occurs in a finite range of velocities-the bouncing window. The fusion velocity is rather independent of the nanoparticle radius.