Two trains, going in opposite directions,
free-wheel past each other on straight, level, railway tracks.
As they do so the passengers throw heavy masses out of their
own train and into the other train. This slows the two trains
relative to each other.
At a sub-microscopic level an object moving
through a fluid might acquire a coating of the fluid
adherent to its sides. This coat and the body of the fluid
exchange particles by diffusion. By analogy to the trains
this tends to slow the relative motion of the body and the
fluid.
For a more general mechanism for the slowing
of the motion see: Non-Dissipative.