ABSTRACT
To produce polarization by extinction, interstellar grains must be elongated or flattened and aligned, at least, some of them. As they are not sitting still but receive stochastically angular momentum in collisions with gas atoms, they spin and it is their rotation axes that have to be aligned somehow, not the grains themselves. This chapter summarizes the tools necessary for understanding the mechanical and magnetic processes. It describes them emphasizing the ideas behind them and not the final numbers which are rather vague. Rotating bodies seem to misbehave in the sense that their motion evades intuition: we push a spinning top one way and it goes the other. However, a child can ride a bicycle with ease, so somehow we can cope with rotation, at least in praxi. When there is equipartition between dust particles and gas, the grains perform translatory and rotational Brownian motion.
