Similar to this, every earthenware vessel that lets in [liquid], lets out [liquid], but there can be [an earthenware vessel] that lets out [liquid] but does not let in [liquid].  Every limb that has a fingernail has a bone, but there can be [a limb] that has a bone but does not have a fingernail.

Every [object] that contracts “treading” tumah contracts corpse tumah, but there can be [an object] that contracts corpse tumah but does not contract “treading” tumah.

Everyone who is eligible to judge capital cases is eligible to judge monetary cases, but there can be [someone] who is eligible to judge monetary cases but is not eligible to judge capital cases.  Everyone who is fit to judge is fit to testify, but there can be [someone] who is fit to testify but is not fit to judge.

Every [food] that is subject to tithes contracts food tumah, but there can be [a food] that contracts food tumah but is not subject to tithes.