Two visitors walk in the Memorial to the Murdered Jews of Europe on the International Museum Day in Berlin, capital of Germany, on May 18, 2017. (Xinhua/Shan Yuqi)

Visitors look on displayed documents at the underground "Place of Information" of the Memorial to the Murdered Jews of Europe on the International Museum Day in Berlin, capital of Germany, on May 18, 2017. (Xinhua/Shan Yuqi)

Tourists visit the National Bardo Museum on the International Museum Day in Tunis, capital of Tunisia, on May 18, 2017. International Museum Day is an occasion to raise awareness of the importance of museums for the development of society. This year, the event will celebrate the theme "Museums and contested histories: Saying the unspeakable in museums." (Xinhua/Adele Ezzine)

Tourists visit the National Bardo Museum on the International Museum Day in Tunis, capital of Tunisia, on May 18, 2017. International Museum Day is an occasion to raise awareness of the importance of museums for the development of society. This year, the event will celebrate the theme "Museums and contested histories: Saying the unspeakable in museums." (Xinhua/Adele Ezzine)

People visit the Egyptian Museum on the International Museum Day in Cairo, Egypt on May 18, 2017. The Egyptian Museum opened for free to all the visitors on Thursday. (Xinhua/Ahmed Gomaa)

Visitors look at the golden burial mask of King Tutankhamun at the Egyptian Museum on the International Museum Day in Cairo, Egypt on May 18, 2017. The Egyptian Museum opened for free to all the visitors on Thursday. (Xinhua/Ahmed Gomaa)

A visitor takes photos of exhibits displayed in the Egyptian Museum on the International Museum Day in Cairo, Egypt on May 18, 2017. The Egyptian Museum opened for free to all the visitors on Thursday. (Xinhua/Ahmed Gomaa)

Visitors take a selfie as they visit the Egyptian Museum on the International Museum Day in Cairo, Egypt on May 18, 2017. The Egyptian Museum opened for free to all the visitors on Thursday. (Xinhua/Ahmed Gomaa)

A visitor looks at the golden burial mask of King Tutankhamun at the Egyptian Museum on the International Museum Day in Cairo, Egypt on May 18, 2017. The Egyptian Museum opened for free to all the visitors on Thursday. (Xinhua/Ahmed Gomaa)