A total bargain and must have for your home! When I first received these items I gave them a quick leak test. Fluffing it open and twisting the lid on, I easily captured air inside like a balloon then gave it a squeeze. They are all literally air tight, not even slow leaks. On top of being adorable, their caps are not shallow but deep, so you can have peace of mind that it won't easily or accidentally come undone and leak. Be sure not to overfill it.. The material itself is thick, possibly double layered. Overall the design of these ice packs is high quality and the price can't be beat. They arrived quickly as well and just in time.