Pope speaks about childhood and early mornings as an altar boy in unscripted visit with campers

The visit, which was not announced in advance, took place in the Vatican's main audience hall, which was decked out with huge inflatable bouncy castles for the estimated 600 kids.

One of the young campers, Giulia, asked Leo if he used to go to Mass as a child. The former Robert Prevost, who grew up the youngest of three brothers in the south Chicago suburb of Dolton, said the family always went to Mass on Sundays.

"But starting from when I was around 6 years old, I was also an altar boy in the parish. And so before going to school -- it was a parochial school -- there was Mass at 6:30 a.m.," he said, emphasizing how early it was. "And Mom would wake us up and say 'We're going to Mass!' Because serving Mass was something we liked because starting from when I was young, they taught us that Jesus was always close to us."

Leo, who was born in 1955, recalled that at the time, Mass was celebrated in Latin. He said he had to learn it to serve Mass even before he made his First Communion, one of the key sacraments in the church.

"It wasn't so much the language that it was celebrated in but the experience of getting to know other kids who served the Mass together, the friendship, and this closeness with Jesus in the church," he said.

Leo's brother, John Prevost, has said his little brother knew from a very young age that he wanted to be a priest. Young Robert used to pretend to celebrate Mass using their mother's ironing board as an altar and Necco candies — a once-popular sweet — as Communion wafers.

History's first American pope spoke in Italian, but he switched to English to address a group of Ukrainian children, some of whom held up Ukrainian flags and snagged Leo autographs. He spoke about the benefits of meeting people from different backgrounds, languages and lands.

It was one of the first times Leo has spoken unscripted at length in public, responding to questions posed to him by the children. He has tended to stick to his prepared texts for his audiences so far in his young pontificate.