New York City policemen tangled with demonstrators at a subway station on the opening day of the New York World's Fair, April 22, 1964. Youths attempted to stall the train, which was headed from the city to the fairgrounds, as a form of protest on behalf of civil rights for blacks. (Photo by Charles Gorry/AP Photo)

Alex Plunkett, left, and Sean Hart, both of Arlington, Va., join hundreds participating in the annual International Pillow Fight Day on April 5. Massive pillow fights broke out on the Mall in Washington and in cities around the world. (Photo by Marvin Joseph/The Washington Post)

Stacked stones are pictured during the European Stone Stacking Championships 2018 in Dunbar, Scotland, on April 22, 2018. The European Stone Stacking Championships are Europe’s largest championships for all Stone Stacking and Rock Balancing artists and practitioners. The overall winner of the European Stone Stacking Championship will receive financial support for flights to the World Stone Balance Championship in Llano, Texas the following year. (Photo by Andy Buchanan/AFP Photo)

Danusorn Sdisaithaworn, 10, poses for a portrait during an annual Poy Sang Long celebration, a traditional rite of passage for boys to be initiated as Buddhist novices, while he visits a relative's house outside Mae Hong Son, Thailand, April 4, 2018. (Photo by Jorge Silva/Reuters)

Mae Zelinsky, left, and Betty O'Beda test ammunition on a .30 caliber rifle at a Remington Arms plant on April 30, 1943. Many women took over jobs that were left vacant when men went overseas to fight in World War II. (Photo by AP Photo)

A fisherman transports a shark and other fish with his ride to the traditional fish market in Lampulo, Aceh, Indonesia, 12 April 2018. The fishing industry is one of Indonesia's largest economic contributor, with almost half of the country's population working in the sector. (Photo by Hotli Simanjuntak/EPA/EFE)

Lily Aldridge and Stella Maxwell arrive at the Daily Front Row's 3rd Annual Fashion Los Angeles Awards at the Sunset Tower Hotel on April 2, 2017 in West Hollywood, California. (Photo by Jon Kopaloff/FilmMagic)

An Iraqi Army member launches a rocket during the operation to retake Mosul from Daesh terrorists in Jadid neighborhood as the clashes between Iraqi Army and Daesh terrorists continue in Mosul, Iraq on April 2, 2017. (Photo by Hemn Baban/Anadolu Agency/Getty Images)