Lucy says to Linus that she hopes to find the Romana Lisa at the art museum that they and the other students from their school are visiting. The correct name for the painting is, of course, the Mona Lisa, and it is on permanent display at the Louvre in Paris.
Any grade school student would quickly realize that they were in a supermarket rather than a museum, especially an art museum. Yet, Charlie Brown, Peppermint Patty, and Marcie remain completely oblivious to that fact.