When it's Monday and you don't want to go to work.

“Dogs show what may be fairly called a sense of humor, as distinct from mere play; if a bit of stick…be thrown to one, he will often carry it away for a short distance; and then squatting down with it on the ground close before him, will wait until his master comes quite close to take it away. The dog will then seize it and rush away in triumph, repeating the same maneuver, and evidently enjoying the practical joke.”