A woman takes part in a four-minute flash mob pillow fight at the Old Town Square in Prague, Czech Republic, December 22, 2016. (Photo by David W. Cerny/Reuters)

An enthusiast dressed as a zombie takes part in the annual Zombie Walk festival in Prague June 1, 2013. (Photo by David W. Cerny/Reuters)

A participant runs through colored powder as she takes part in the Get Rainbowed run in Prague May 23, 2015. Get Rainbowed is a five-kilometre race with the aim of promoting a healthy lifestyle. (Photo by David W. Cerny/Reuters)

The Prague astronomical clock, or Prague orloj, is a medieval astronomical clock located in Prague, the capital of the Czech Republic. The clock was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest one still working.

Participants wearing historical costumes ride their high-wheel bicycles during the annual penny farthing race in Prague, Czech Republic November 5, 2016. (Photo by David W. Cerny/Reuters)

The Clementinum (Klementinum in Czech) is a historic complex of buildings in Prague. Until recently the complex hosted the National, University and Technical libraries, the City Library also being located nearby on Mariánské Náměstí. The Technical library and the Municipal library have moved to the Prague National Technical Library at Technická 6 since 2009. It is currently in use as the National Library of the Czech Republic. In 2005, the Czech National Library received the UNESCO Jikji prize (Memory of the World).

A child looks at a polar bear swimming under a water in its enclosure on a hot summer day at Prague Zoo, Czech Republic, July 30, 2015. (Photo by David W. Cerny/Reuters)