Our guys did that sack thing, too, once.

Jeff saw Binky FLYING up the basement stairs, then came Archy with the handles of a plastic bag around his neck. He was running so fast that the bag ballooned out behind him and he looked like the Caped Crusader! Both were scared silly (Binky because he's a scardey-cat, and Archy, because the bag crinkled).

Now we keep all bags, plastic or whatever, safely away!