It's pretty funny when the cat explodes like a terrified watermelon and sprints off into the bushes, but the best part is the couple of seconds before, when, subtly interrupted from gazing at whatever it is that cats gaze at, it's all like, "hello, what's this?" You can tell right at that point that the cat is already mildly annoyed before anything has even really happened, and that is what makes the ensuing moment so priceless.
Seriously, though, it's somewhat understandable: If I had to live with a couple of ass-hats who constantly giggled like the joker on helium, I'd probably be constantly annoyed, too. Nice work, dudes, but it's not like annoying a cat is a giant accomplishment.