We believe that planets form in a disk (which contains gas and dust) around young stars. Very small (micrometer) sized dust collide with each other mediated by turbulent gas flow. These collisions (in some cases) can give rise to coagulation by which the dust grains grow. A similar process of collisional coagulation is thought to be responsible in formation of raindrops from minute sized water droplets in clouds. One part of the process is determined by turbulence (rate of collisions, relative velocities during collisions, etc) and the other part (merging, bouncing, fragmenting) depend on the material properties of dust (or water droplets). If all collisions give rise to coagulation, we get into the regime of Smoluchowzki equations which also describes the curdling of milk. I shall spend a significant time discussing the different aspects of this many-splendoured problem and summarise our recent results.