In the early stages of planet formation dust grains collect to form km-sized planetesimals. Growth beyond a few cms is frustrated by collisional shattering and rapid radial drift of larger particles. I will present my work on the dynamics of pebbles, rocks, and boulders in gaseous protoplanetary disks. The loose drag force coupling with the gas is associated with a clumping instability which makes particles move in dense clumps, similar to how migrating geese and bicycle riders travel in groups. The local density of rocks and boulders becomes high enough to allow a gravitational contraction into gravitationally bound clusters with masses comparable to solid objects of several hundred kilometers in diameter. As 100-km scale protoplanets may form in an environment dense in dust and pebbles, the accretion of small solids is important for their further growth. I show that accretion of pebbles in a gaseous environment is very efficient and that the process leads to prograde spin up of the growing protoplanets, in agreement with the trend for prograde rotation displayed by the largest asteroids.
