Q: Rabbi Maroof,
When opening a can of tuna on Shabbat, am I making a keli (new vessel)? Same question for opening a bottle of soda with a plastic twist off cap. Is this a problem?
A: Dear Steve,
With regard to soda bottles, there is no problem at all. The cap is designed to be removed - this is why it has a perforated ring on the bottom.
Tuna cans are more complicated because there is no preexistent "opening." The most common approach to this problem is to puncture the underside of the can prior to opening it; this renders the can unusable and thereby ensures that one is not fashioning a vessel by removing its top. The top of the tuna can itself is definitely not considered a vessel.