There are some military issued items that are perfect and some that are not so perfect. The USGI 1-Quart Canteen System is by far the best canteen system ever fielded by any army and the canteen cup is one of the main reasons. With the perfect size, shape, and features to allow it to pack and store easily you'll become a convert from the first time you use one.
While the main purpose of the US 1-Quart Canteen Cup is to provide a method of drinking the stored liquid, it has two roles. When not being used, the 1-Quart Canteen will nest inside of the canteen cup. This space saving design will provide a layer of protection to the canteen while you are in the field and ensure that when you need your canteen it is ready for you.