If you have a flask equiped, you are able to drink from other flasks or cups inside containers/backpack/inventory.
Hope this is intended, decided to share this very nice useful find even if it is a bug.
Hope i do not regreat it.
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Trying to sip from a flask why sifting with a prospectors pan makes it wigg out for a few seconds, glitching between the two.
Was never sure if being able to sip from a flask and continueing to work was ever intended. (can sip when cooking, mining etc etc, without interuption)