Most people associate wagging tails with happy dogs, but I'm betting not many pay attention to which direction that tail moves. Now, obviously while wagging, it goes back and forth but according to this study, the "striking asymmetries in the control of tail movements" can tell how the right and left halves of the brain control different emotions.
Next time you see this, check which direction the bias is – the wagging should be more pronounced in one direction like bouncing an eraser-topped pencil on a table. Tails wag to the right when dogs are happy and see something they want to approach, and to the left when they are frightened and confronted with something they want to run away from.
Go forth and find waggy tails and lemme hear all about your findings in the comments below.