When Joe Deedon teaches teachers and school faculty how to handle an active shooter — the scenario no one wants to imagine, let alone rehearse in mental drills — his guidance depends on the size of the children. In a high school classroom, he tells the teachers, the biggest students, maybe football players, may be able to help fight, when fighting is the only option left.

In a middle school, the children are big enough to push desks against the door to help keep the shooter out.

Younger than that? Very small ones, he says, can at least run around throwing objects if the gunman enters the room. "Try to overwhelm him," Deedon teaches, "throw him off his plan."

The image is unthinkable: small children, preschoolers even, pitching blocks and books at a shooter intent on harming them. That someone even prepares teachers for such a scenario — Deedon, a former sheriff's deputy, runs a company out of Denver that trains schools and law enforcement on active shooters — is deeply unsettling, too.

"It’s unfortunate that it’s come to that," Deedon says. "It’s the only thing we can do outside of building our schools like prisons. If you want to be warming, welcoming, inviting, conducive to learning, you can’t have it so secure. And we don’t have the money for that anyway."

This is the reality, after Columbine unnerved high schools, and Virginia Tech college campuses, and Mumbai the hotel industry, and Aurora movie theaters, and Gabrielle Giffords shopping centers: So many people, even kindergartners, now go through some kind of preparation for the tiny but terrifying possibility of an active shooter, which has the effect of reminding us we live in a world where that might happen — anywhere.

"For schools, Sandy Hook was a major game-changer for them," says Pete Blair, the executive director of the Advanced Law Enforcement Rapid Response Training Center at Texas State University, referring to the massacre at a Connecticut elementary school in late 2012. "That’s when most schools sat up and paid attention."

Now teachers learn that when they close and lock the classroom door, they must leave it locked, even if a student pleads from the hallway. "That’s a very hard thing," Blair says. But the most likely attacker in a school shooting, he says, is a student. Some states post-Sandy Hook now require schools to conduct active-shooter and intruder drills, where scenarios like this are tested. Virginia added lockdown drills, and Washington upped their frequency to four a year.

Deedon's company, TAC*ONE Consulting, primarily trains the law enforcement that trains local communities. But, in addition to schools, they've worked with hospitals and private companies, and a casino resort on the Las Vegas strip, with courses that range from 45 minutes to four hours for as much as $75 a person. After Wednesday's shooting in San Bernardino, Calif., that same afternoon, Deedon heard from two private-sector organizations, one school district and a college campus. "Whenever these things happen," he says, "my phone and my emails — it’s unbelievable."

Last year, the National Retail Federation conducted a study of whether consumers even know what "active shooter" means (nearly half do) and if they understand what they'd be told if a scenario occurred (about three in five recognize "shelter in place"). For organizations and law enforcement preparing for an attack, including retailers, that awareness means many of us may have heard something about how to behave.

But it also means that the hypothetical active shooter looms ever larger in our lives.

"That’s the big question before us: What are we going to let fear do to us, and what aren’t we? That’s our choice," says Hope Jahren, a scientist at the University of Hawaii at Manoa. Her son has had lockdown drills since kindergarten, which remind her of the nuclear drills she knew as a child during the Cold War.

"What I remember feeling was like, 'The grownups, the powers that be, it’s your responsibly to make peace with Russia, and you’re telling me the answer for this is for me to hide in the basement with a jug of water for 24 hours?'" she says. "It’s crazy. It's a weird way of shifting responsibility."

Critics of all this modern preparation make a similar point today: that we teach our children to huddle in closets rather than changing our gun laws.

Bill Worth, the managing partner of Countermeasure Consulting Group, insists that awareness will create more safety. His firm, based out of the Chicago area, offers site assessments to companies to spot their vulnerabilities and then training for the active shooters who might exploit them ("When seconds count... what's your plan?" the company's website says). A small firm might pay $5,000, a big one $20,000. He's also working now on a book tentatively titled "Your church: the softest target in America."

To think about such things — especially for a living — is to view nearly everywhere as a would-be target. Each time he enters a building, Worth looks for the exits. When he walks into a restaurant, he surveys the diners. When he goes to church, he lingers in the foyer watching who else comes in.

"It’s invigorating and it makes me stronger," he says. "Because there are bad people in the world. They want to hurt you, they want to hurt me. I pray it’ll never, ever happen. But I don’t want to be unprepared for it."

His business, he acknowledges, wouldn't have existed 10 years ago. It's the byproduct of rising  attacks, a response to rising fear. "Ten years ago, we would have never talked about having armed security in the church," he says. "Who would have ever thought somebody would come into a Christmas party with AR15s and kill 14 people. That would have been so foreign to us."

Even in Canada, a country without America's closely held gun culture and frequency of gun violence, active-shooter drills are happening, too. Nadia Prigoda-Lee lives in the town of Charlottetown on Prince Edward Island. Earlier this fall, her 5-year-old came home from a drill in her kindergarten class.

The girl recounted that she had done a good job when everyone in class went to the special place and the teacher turned off the lights and she was quiet. She was so proud. Her mother later cried.

"I was thinking about the parents that this really has affected directly," Prigoda-Lee says, "hoping that I’ll be lucky and it won’t happen for us. And then I was also thinking about how this represents a real loss of innocence for my daughter."