Audience watch a performance in hot tubs on the roof of a building in Prague, the Czech Republic, on Aug. 29, 2019. Czech opera singers sang the most famous aria of Don Giovanni on the roof of a building in Prague on Thursday. (Photo by Dana Kesnerova/Xinhua)

Audience watch a performance in hot tubs on the roof of a building in Prague, the Czech Republic, on Aug. 29, 2019. Czech opera singers sang the most famous aria of Don Giovanni on the roof of a building in Prague on Thursday. (Photo by Dana Kesnerova/Xinhua)

Audience watch a performance in hot tubs on the roof of a building in Prague, the Czech Republic, on Aug. 29, 2019. Czech opera singers sang the most famous aria of Don Giovanni on the roof of a building in Prague on Thursday. (Photo by Dana Kesnerova/Xinhua)

Audience watch a performance in hot tubs on the roof of a building in Prague, the Czech Republic, on Aug. 29, 2019. Czech opera singers sang the most famous aria of Don Giovanni on the roof of a building in Prague on Thursday. (Photo by Dana Kesnerova/Xinhua)