How do bees put the pollen into the pollen sacs?

When bees are collecting pollen, the grains stick to specialized hairs around the knee joint of their hind legs, forming sacs. After an hour of collecting a build up of pollen grains, usually yellowish in color, is quite visible on the hind legs. The fuzzy hairs, stickiness of the pollen and electrostatic electricity contribute to the build up of these sacs.