A boy covers his head with fallen leaves as he enjoys a stroll with his family in a park in downtown Frankfurt, Germany, November 6, 2016. (Photo by Kai Pfaffenbach/Reuters)

Border between Sweden and Norway at Moldusen. An approximately 20-meter wide clearing in the forest separates the two Scandinavian nations, consequently cutting Finnskogen in two. Grue Finnskog 2016. (Photo by Terje Abusdal/The Washington Post)

View of Evolutions installation by artist Yann Nguema at the Saint-Jean Cathedral during the rehearsal for the Festival of Lights (Fetes des Lumieres) in Central Lyon, France, late December 7, 2016. (Photo by Emmanuel Foudrot/Reuters)

A clown poses for a picture in a parade during Salvadoran Clown Day celebrations in San Salvador, El Salvador, December 7, 2016. (Photo by Jose Cabezas/Reuters)

A visitor watches the exhibit “Enchanted: Forest of Light” at Descanso Gardens in La Canada Flintridge, California U.S., December 9, 2016. (Photo by Mario Anzuoni/Reuters)

A girl wearing a mask takes part in a procession celebrating the religious holiday of Mawlid al-Nabi, the birthday of Prophet Mohammad, in Benghazi, Libya December 10, 2016. (Photo by Esam Omran Al-Fetori/Reuters)

Members of the Iraqi Army fire towards Islamic State militant positions at the south of Mosul, Iraq December 10, 2016. (Photo by Alaa Al-Marjani/Reuters)

Costumed participants take a break during a traditional Perchtenlauf (Perchten parade) in Osterseeon near Munich, Germany, December 17, 2016. (Photo by Michaela Rehle/Reuters)