In ancient times, when roads were bad and footwear was worse, the washing of a guest's feet was a required sign of hospitality. Today when someone comes to your home, you're more likely to offer to take their coat and bring them beverage rather then have the help fetch a basin to refresh their worn feet.
The gesture of a servant's washing a newly arrived guest's feet is sprinkled throughout the Jewish and Christian scriptures. That the characters in question were respectable, hospitable, and well off would have been culturally recognizable to earlier readers. In the Christian tradition, one story of feet washing entirely changed the paradigm.
In the Biblical accounts of the Easter story, Jesus rides into Jerusalem on Palm Sunday on a donkey to adoring crowds. Just a few days later he gathers his 12 disciples for what would be their Last Supper before he was crucified.
According to the Gospel of John, after everyone has reclined at table before the meal, Jesus, dressed as a servant, washes their feet. The disciple Peter recoils at the sight of his master taking on the role of a servant.
Jesus then explains to them, "So if I, your Lord and Teacher, have washed your feet, you also ought to wash one another's feet." (John 13:15 NRSV)
Christians have carried on this practice of role reversal for the past two millennia, particularly during Holy Week. On Holy Thursday believers commemorate the Last Supper, where after he washed the disciples' feet, Jesus went on to institute the practice of Communion -- breaking bread to symbolize the sacrifice of his body for his followers to atone for their sins, the core of Christian belief.
Thursday in Rome, Pope Francis continued a long papal tradition when he participated in the foot-washing rite.
In a radical departure from tradition, Francis said Holy Thursday Mass not at the ornately appointed Basilica of St. John Lateran, but in a small chapel with nearly 50 teenage boys and girls at Casal del Marmo Penitential Institute for Minors, a juvenile detention center. The 76-year-old leader of a more than 1 billion Catholics called up 12 of the young inmates from different nationalities and "diverse religious confessions," according to the Vatican. He will stoop down before them, one by one, with a basin of water and wash their feet.
This is the continuation of a personal custom; when he was Cardinal Jorge Mario Bergoglio, the archbishop of Buenos Aires, he famously washed the feet of AIDS patients and drug addicts.
Those who know him are not surprised that on Holy Thursday he looks to the poor and marginalized.
"Really, what he's doing is very much in character," said the Rev. Thomas Rosica, who has served as a Vatican spokesman during the papal transition.
Rosica said the timing of the election also factored into why the new pope selected a prison over a cathedral.
"Technically, he couldn't celebrate the Mass at St. John Lateran because he hasn't yet taken possession of the cathedral." But it is a fair assumption that had he wanted to have Mass there, it would have happened.
For several years Rosica lived in the Middle East, where foot-washing is still done, "and it's the lowliest person in the house who does that as a sign of great respect."
"There's something extremely intimate about that, when you're touching someone's feet," he said.
Rosica said that with the ceremony, the pope is applying biblical texts to concrete actions.
Pia de Solenni, an ethicist and moral theologian, said the significance of Jesus Christ washing the disciples' feet resounds over the centuries: "At that time and now there's that sense of hierarchy, or that clergy are set apart, that they're somehow preferred. What Jesus is teaching and what I think is the authentic understanding of the priesthood in the Catholic Church is that the priest - your teacher, your leader - is first and foremost a servant," she said.
"That's really turning the worldly understanding of who a leader is upside down."
The ceremony for the young prisoners is another in a growing list change brought by Francis. After his election this month, he refused the papal limo and rode the bus with the other cardinals.
On Tuesday the Vatican announced Francis had opted to stay in room 201 at Case Santa Marta, the Vatican guest house, instead of moving into the palatial papal apartment.
For his inaugural Mass and again on Palm Sunday, he opted for an open-air "popemobile," foregoing the safety-glass-enclosed version that became the vehicle of choice for public processions following an assassination attempt on Pope John Paul II.
The changes have, in part, led to overflow crowds for the new pope's public appearances and headaches for his security detail; he has shown he is willing to break out of the bubble to greet the faithful and kiss seemingly every baby in sight.
"He's making a very special place for the poor, the disenfranchised," Rosica said. "He's kind of taken the world by storm by the symbols and the signals he's given."
Irwin, a professor at the Catholic University of America, told CNN from Rome that foot-washing "has always been - for priests, bishops and popes throughout the centuries - a role reversal. That is to say, those who are marginalized become center stage and the one who is usually center stage washes their feet."
Irwin said Francis' choice to wash prisoners' feet fits in with a broader narrative of the new pope's journey. Whereas his predecessor, Benedict, came to the papacy by way of the scholarly library and high Vatican offices (Rosica called him "the Mozart of theologians") Francis "has a pastor's heart."
"In that sense he's probably more of a journeyman than he is a highly accomplished scholar," Irwin said. "In the process we'll all get on board with the journeyman." And journeymen, he said, "make the world work."