We once had a neighbor whose hyperactive Maltese terrier would chase a flashlight beam around the room for as long as we had the energy to move the light. His seriousness in attempting to capture what was obviously a highly annoying if not downright dangerous prey never failed to amuse. Nor does the following:
Link via Neatorama

You know,Eric, I just don't trust those ugly little creatures with their beady eyes!
But you have to admit that they are valuable in eating actual pests. Ugly, true, but still valuable.
That's hilarious!
We used to torment our cat with a laser pointer. What was really funny was when we'd let her "catch" it.
We were always afraid to let the little dog "catch" the flashlight beam, fearful of ruining the magic for him. We had already inadvertently let slip that there was no actual Easter bunny, and that was all the emotional trauma we could handle.