Drink juice from a large 1 litre packet while sitting in a conspicuous glass enclosure where everyone who walks by will see you, and then deny you were drinking at all when a librarian informs you that no eating or drinking is allowed in a library.
See photo here.
How to behave…
I have an idea for how the librarian could behave. Let people have (covered) drinks in the library. Problem solved!
:o)