When choosing your caps, remember to consider if the can you are using has a female or male valve? A can with a female valve has no plastic valve stem protruding out of the top of the can. This is the most common of cans and only takes male caps that have a small valve stem that enter the can. A can with a male valve has a plastic stem protruding out of its top, and thus can only be used with female caps. Click below to learn the traits and characteristics of each cap.