F is for Friday! Wait a minute...
F is for Flexible prims
Flexible prims were brand new to the grid when I signed up back in spring 2006. They're used for everything from flags and hair to tentacles and ballgowns.
Natalia's is still one of the best tutorial on using flexi prims.
If you want to make a more complex shape flex, it's possible to group flexi prims into units that move as one, although there are a lot of restrictions on making that work.
Flexi properties can be manipulated with lsl using the PrimitiveParams() functions.