Hjalmar used to wag side-to-side with his entire tail, and when he did it
the most was during great pleasurable excitment -- with the best being a
rousing game of "Where's Hjalmar? -- a personalized form of hide and seek.
What he never DID realize, though was that just because he couldn't see me
didn't mean that I couldn't see him.  so he'd often hid with his head and
thorax under the overhanging bedcovers while his pudgy bum and wagging tail
were hanging out.  By rules of the game I had to keep questioning, "Where's
Hjalmar?" over and over till he glimpsed out and then exclaim definitively,
"There's Hjalmar!" You can imagine how hard it was to not break down in
laughter.
 
We've had others who wagged only in tubes.  I think they just enjoyed
creating a racket -- kind of like the joy they get from dropping things
from heights.  Granted, they could have started out judging the area around
them with sensory feedback form their tails, but later it was just fun drum
roll time.
[Posted in FML issue 2380]