Few people have ever been inside that world, but Lucy Walker has.
"They'll wear big black bonnets. Inside those outfits, they're little girls like anyone else," says Walker, who's been inside the Nickel Mines schoolhouse, which became a crime scene on Monday.
"It's a tragedy that it struck them in particular because they go to such lengths to avoid violence," she says.
Walker gained access into the secretive community while filming her documentary about the Amish. Normally, peaceful, quiet and religious people, her film, "The Devil's Playground" focuses on a tumultuous Amish ritual called "rumspringa."
When the Amish turn 16, they are allowed to explore the customs of the outside world.
"Some of them will pack in a lot of hijinks because once they settle down, the gates close," Lucy says.
And they'll have to decide what side of the gate they want to be on. The teens eventually choose whether to join the Amish church or leave it altogether. Walker says 90 percent join.
"You feel it within you. The thought that you're different from other people is so deep-seated," Lucy says.
So they work on their family's farms until they marry. Men grow beards. Women never cut their hair.
The Amish believe their plain, distinctive clothes encourage humility and separation from the world, with all of its violence and pain. Monday, they learned that separating from the world isn't always that simple.