How? [If] one’s hands were tahor and the exterior of the cup was tamei, [if] he held it by its tzeviah place, he need not be concerned that perhaps his hands were rendered tamei by the exterior of the cup. [If] one was drinking from a cup whose exterior was tamei, he need not be concerned that perhaps the liquid in his mouth was rendered tamei by the exterior of the cup and then rendered tamei the [interior of the] cup. [Regarding] a kettle that is boiling [water], one need not be concerned that perhaps liquid left it, touched its exterior, and then returned to its inside.