Curious cats love to get inside a lot of different things, but they have a weakness for boxes and bags. A box, warm and snug, gives cats a sense of comfort and security. A paper bag or any type of bag probably has the same effect on cats, with an added entertainment value if the bag makes rustling sound.
Cats also love to stick their heads inside any kind of vessel. They become evidently excited as they tunnel deep inside to “explore”. If you have kitties, they probably gave you already several demonstrations of their box and bag obsession.
Here are our photos of funny cats having a blast stuffing themselves inside anything they find. It's amusing to watch cats get lost in their “new” world, but stick around to ensure they don't suffocate.