We took a hop on-hop off bus to The Louvre Museum, the world's largest art museum. It contains more than 380,000 objects and 35,000 works of art dating from 4000 BC to the 21st century. It is housed in the Louvre Palace, originally built as the Louvre Castle in the late 12th to 13th century. It is the world's most visited art museum, receiving more than 10 million visitors every year. Its most popular attraction is Leonardo da Vinci's painting, Mona Lisa, which has been called the best known, most visited, most written about, most sung about and most parodied work of art in the world. It's kept in its own gallery with two guards and we had to elbow through a sweaty mob of people to get close enough to take a picture.