About the only thing my cats like more than boxes, are paper bags. After I get groceries, I leave the paper bags on the floor and they climb in them.
Usually one inside will start pawing at the paper on the bottom of the bag, and the other one will paw back at the movements from outside. They get pretty wild sometimes, it's so much fun to watch.
The store we shop at has handles on the bags and I learned pretty quick to cut the handles off before I set them out for play. My red-head got her head stuck in the handle trying to get out once and was stuck.
A cat moving around in the bag also draws the attention of another cat, who will sometimes just plain pounce on the cat in the bag. Once after playing for a bit, one cat pounced on a cat in the bag, then was so tired she went to sleep. Her sister must have been just as tired because she just laid in there with her nice warm cat quilt on top of her and went to sleep, too.
When I take them to the vet (only 4 blocks away) I don't bother with the carriers anymore. The carriers make them nervous. I just set out bags, they climb in, and I grab the handles and take them that way. It goes a lot smoother
The first time we did it we had 4 cats and it sure got a chuckle out of everyone in the office when they started poking their noses out of the bags. The cats saw all the dogs and ducked back inside and got as small as they could.
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