Michael Craig-Martin’s 1973 conceptual artwork An Oak Tree presents a glass of water with a plaque explaining that it’s a tree — not symbolically but literally: “The actual oak tree is physically present but in the form of the glass of water.”

This is a comment on transubstantiation and, by extension, on the patron’s faith in an artist’s presentation of his work, but it backfired: When the National Gallery of Australia bought the piece in 1977, customs officials barred it as “vegetation.”