Throughout the solar system, rocks are flying. We call them meteorites, until they hit one of our spacecraft, when we call them substantially less polite things. You are experts on these fascinating lumps of rock. You regularly travel to Antarctica to collect meterorites, you study them under the microscope, and you also work with big guns which fire rocks into one another, to investigate the processes that take place when meteorites collide with each other. If, however, the grains are larger, something very different starts to happen.
Grains start hitting each other and sticking together, so they rapidly grow in size.