Long ago I bought a postcard at the Centre Pompidou Museum of Modern Art in Paris. I bought it because of an image of Mona Lisa on it. The title was "Utiliser un Rembrandt comme planche a repasser, Marcel Duchamp" by Daniel Spoerri. I wondered why it was Mona Lisa, not a Rembrandt, on the ironing board (planche a repasser) in the image.
This made me uneasy. I felt that no artist could be ignorant about who painted the Mona Lisa. Then I discovered that I was the ignorant one -- but ignorant of something much more obscure than who painted the Mona Lisa. You see, Marcel Duchamp made notes about various Dada art topics. Once he talked about using a Rembrandt for an ironing board. That obviously inspired the artist, who referred to Duchamp indirectly by the image of Mona Lisa. I applaud this Dada indirection!
Everything was interesting to Marcel Duchamp, as reflected in his readymades, so who knew what he might be thinking about an ironing board. One of my favorites among the less-well-known is his version (maybe versions) of the Parisian sign "Eau et Gaz a tous les Etages," or just "Gaz..." as shown in the photo from a Paris street.
The sign on a Paris building means that there is water and gas service on every floor. When in Paris, I found these placards mysterious, and was told they were there to inform the fire department. They date from the era when not all buildings had water and gas on every floor. Marcel Duchamp suggests that they are absurdities, like so many texts you see on the street.
Once I dreamed that Marcel Duchamp and I were walking along and he said "I am the eau et gaz." At the moment of waking up I had a clear insight: Marcel Duchamp was water! and air! Elemental.