Egyptian bootblacks cleaning a European's shoes in Cairo. (Photo by Hulton Archive/Getty Images). Circa 1870

The Hubble Space Telescope is shown following its release from the space shuttle Discovery Wednesday, February 19, 1997. The Hubble Space Telescope, one of NASA'S crowning glories, marks its 25th anniversary on Friday, April 24, 2015. With more than 1 million observations, including those of the farthest and oldest galaxies ever beholden by humanity, no man-made satellite has touched as many minds or hearts as Hubble. (Photo by AP Photo/NASA)

A Nepalese man sits outside a Hindu temple as a goat rests on a parked scooter in Khokana, Lalitpur District, Nepal, Monday, September 26, 2016. Writing on the wall offers prayers to Hindu god Shiva. (Photo by Niranjan Shrestha/AP Photo)

Ukrainian servicemen patrol near the humanitarian aid center in Avdiivka, Ukraine, Saturday, February 4, 2017. Fighting in eastern Ukraine sharply escalated this week. Ukraine's military said several soldiers were killed over the past day in shelling in eastern Ukraine, where fighting has escalated over the past week. (Photo by Evgeniy Maloletka/AP Photo)

Nuns react at the arrival of Pope Francis at the St. Mary's school to attend a meeting of clergy and religious in Nairobi, Kenya, 26 November 2015. Pope Francis is on a six days visit that will take him to Kenya, Uganda and the Repulic of Central Africa from 25 to 30 November. (Photo by Daniel Dal Zennaro/EPA)

In this October 21, 2015, file photo, a young man rides a hoverboard along a Manhattan street toward the Empire State Building in New York. More than 500,000 hoverboards are being recalled after reports that they can burst into flames. The Consumer Product Safety Commission said Wednesday, July 6, 2016, it has received 99 reports of battery packs in the two-wheel motorized scooters catching fire or exploding that causing burns or property damage. The recalled hoverboards were made by eight companies. (Photo by Kathy Willens/AP Photo)

A P-51 Mustang named “Charlotte's Chariot” from the Southern Heritage Air Foundation flies past a cloud of smoke and fire from a nearby battle reenactment during the Wings Over Houston Airshow in Houston, on Saturday, November 1, 2014. Battle reenactments are part of the yearly show held at Ellington Airport. (Photo by Kar Hlava/AP Photo/Bay Area Citizen)

A walruses at Hagenbeck Zoo in Hamburg, Germany on July, 7, 2015. (Photo by Werner Struss/Startraks action)