Philadelphia -- The ruddy-cheeked extrovert is Elder Rodney Mills, age 20. He raps on doors - hundreds every day - with an eager ''knock, na-knock, na-knock-knock'' knock.
The tall, quiet one is Elder Robert Keach, also 20. He goes for a more restrained ''knock na-knock-knock'' knock, but moves just as swiftly as his partner.
They are young men on a mission in Philadelphia.
Far from home, cut off from their families, working seven-day weeks and living in the neighborhoods they evangelize, Keach and Mills form one of 60 teams of young Mormon missionaries knocking on doors in some of the region's poorest neighborhoods.
Today the young ''elders'' - the traditional form of address for Mormon missionaries - are in West Philadelphia.
''Who is it?'' a female voice calls from a rowhouse.
''Elder Keach,'' he replies. ''From the church.''
It is an ambiguous reply, but one that opens doors, even in dicey neighborhoods, to the ''church boys,'' as they are sometimes called.
She opens the door and scans his name tag, his necktie, his short-sleeve white shirt. ''I'm an atheist,'' she says, and closes the door.
Keach rolls his eyes. ''She had a picture of Jesus on the wall,'' he says.
Theirs is the Church of Jesus Christ of Latter Day Saints, one of the world's fastest-growing faiths in the nation, thanks in part to the thousands of young men such as Mills and Keach who are called, at 19, to serve two years as missionaries anywhere in the world.
Although all young Mormon men of good health and character are expected to serve, only about 20 percent do so.
''Hey, man, did I wake you?'' Mills asks a groggy-looking man of about 40 who answered the door in his undershirt. The man nods. It is shortly past noon. He shuts the door abruptly.
Mills and Keach have been on this street several times during their 10 weeks together, and greet some residents familiarly.
''Tell your mom the church guys are here,'' Keach tells a girl who answers one door. She returns to say, ''She's busy.''
''OK,'' Keach says. ''Tell her we'll be back.''
A 60-ish woman in red leotards explains that this is not her house. ''The owner's not here,'' she says.
A boy pops his head out the upstairs window. ''My mom's at work,'' he yells. Minutes later they see her at the same window.
''We're Catholic,'' one woman explains. ''That's cool,'' Mills says. ''Can we pray with you?'' She hesitates, then shakes her head.
''Where's your buddy?'' he later asks a boy of about 12. ''He's in jail,'' the boy replies. ''He tried to kill his mother and grandmother.''
And so it goes. Dogs bark. Bees buzz. No one answers.
Then, a bare-chested teenager answers the door. It does not look promising: He is wearing a rosary around his neck. But he invites them in. Armando, 17, explains in Spanish that he does not have a church, but hopes to find one - and a job. He arrived three days ago from Puerto Rico. He accepts their offer to pray with him, and to come back in a week to learn more.
Founded in upstate New York in 1830 with just six members, the Church of Jesus Christ of Latter-day Saints reached 1 million members in 1947.
Today it numbers nearly 13 million worldwide, and adds a million members every four years.
Its scriptures include the Old Testament and New Testament, as well as the Book of Mormon: a 531-page text that church founder Joseph Smith is said to have translated from hieroglyphics on gold plates he said were shown him by an angel.
Despite the skepticism of most other Christian churches, devout Mormons believe theirs to be the one, true ''restored'' church of Christ.
And with 5.5 million members in the United States, where it is growing at a brisk 1.7 percent annually, the church is now the nation's fourth-largest religious body - bigger than the Presbyterian and Episcopal denominations combined.
Lately, some of its most impressive growth has been in Philadelphia and the adjacent Pennsylvania suburbs, where membership has swollen more than 20 percent since 2004, to about 10,000.
Brant Olson, president of the Philadelphia Stake (a geographical district equivalent to a diocese), attributes some of that growth to new church buildings and an improved ministry to Latinos.
But he gives credit to young missionaries such as Robert Keach and Rodney Mills, who leave behind families and the things of youth to traipse through housing projects and climb rowhouse steps.
''It's been a life-changing experience,'' said Mills, who was called from Salt Lake City. ''The thing I learned most is that when we live according to higher morals and higher standards, we become happier people.''
The two share a plain, two-room apartment on West Cumberland Street, with no computer, TV or radio.
In their 10 weeks working here together, they have brought four families to conversion and baptism.
They awaken at 6 a.m., pray and read Scripture, and start knocking on doors at 10 a.m. They take a break from 3 to 5 p.m., then make house visits with new and prospective converts until 9 p.m.
They spend Sundays at church and Sunday school with new and prospective converts.
''I realize now how easy high school was, and how much time I wasted before on things like TV and videos,'' Mills said.
Both left girlfriends behind, but have no contact with them and do not date here. They don't fly home for weddings or funerals, and are allowed to call home twice a year, on Christmas and Mother's Day.
Keach, who hails from Houston and plans to be a doctor, calls his 20 months in South, West and North Philadelphia ''incredible.''
''It's given me the determination to say I'll never go down the wrong path, because I've seen how fast people can fall when they disobey the commandments or do things that are wrong,'' he said. ''You see it in families. You see it in whole parts of the city.''
Mills, who wants to go into law enforcement, has worked 22 months with about 14 companions in Northeastern Pennsylvania and Philadelphia, but the highlight came in Wilmington.
''A guy opened the door and punched my companion,'' he recalled.
Four days later, the man saw them in the street, apologized, hugged Mills' partner, and joined the LDS church.
''He said he realized he had to change his life,'' Mills said. ''It was really cool.''