Some kids can't sit still for long. They have a hard time paying attention to just one thing. They're easily distracted .They can get very impatient. They hate standing in line or waiting for their turn in a game or activity. They get bored pretty fast. They may also be impulsive--saying the first thing that comes to mind or interrupting someone else who's talking. For certain kids, this problem is so severe  that doctors have a name for it: attention deficit hyperactivity disorder, or ADHD. Perhaps as many as 1 out of every 20 kids under the age of 18 have characteristics of ADHD. Often, these kids have trouble getting through school and face other difficulties later in life. Rizzo started developing the Virtual Classroom in 1999.He wanted to see if he could use it as a tool for testing and treating kids who have attention disorders. To diagnose  ADHD, doctors typically test patients by giving them tasks that require attention. As part of one classic test, you watch letters flashed on a computer screen. Every time you see the letter "A" followed by the letter "X",you have to press the space bar. If you're paying close attention, you'll register all the times this combination occurs. If not, you'll miss some. The Virtual Classroom makes these tests more efficient, Rizzo says. In one experiment, he gave a group of kids the classic "A-X" test. Instead of looking at a computer screen in a doctor's office, though, the kids wore headsets that made it look like they were taking the test in a classroom. "Basically what we found,"Rizzo says,"is that, in 20 minutes of testing with virtual reality,we replicated  a finding that usually requires a couple hours of standard testing with computer screens in the psychologist's office." In the experiment, patients need to press the space bar, when   _  .
see letters A following X first see A then see B see letter X and A see letters A followed by X