At 11:10 a.m. April 20, 1999, Eric Harris and Dylan Klebold arrived at Columbine High School in the suburban town of Columbine, Colo., armed with two shotguns and two pistols. The two entered the school and opened fire; three hours later 15 people were dead.
The shooting was the deadliest to take place in a high school in American history and the name "Columbine" entered the lexicon as a synonym for young people who turn weapons on their fellow students.
The massacre sparked a series of debates that reverberated in the culture from the halls of Congress to the aisles of movie theaters in films like Michael Moore's "Bowling for Columbine" and Gus van Sant's "Elephant."
What drove Klebold, 18, and Harris, 17, to kill 15 people -- including themselves -- and injure an additional 24 sparked a nationwide conversation.
The shootings made many Americans realize the schools they sent their children to every day were not as safe as they seemed, leading to a new movement in school safety.
Everything about the shootings and shooters was dissected in the press, launching a series of debates on topics ranging from gun control to the influence of violent movies and music on young people.
If Roe vs. Wade sparked a national debate about when life begins, the case of Terri Schiavo ignited a countrywide conversation about when it should end.
The case of a brain-dead woman on life support for 15 years and the fight between her husband and parents over whether to keep her on a feeding tube began in a Florida hospital room in 1990 and found its way to the floor of the Congress in 2005.
Eight years after his wife's heart attack in 1990, Michael Schiavo petitioned to remove her feeding tube.
After seven years of wrangling in court, the Florida government and later the federal government passed laws that sought to overturn court orders to remove her feeding tube.
The case became a national cause celebre when Congress held a special session in March 2005 and passed a bill calling for a federal review of her case. President Bush cut short a vacation to return to Washington to sign the bill.
An ABC News poll from that time found that Americans broadly and strongly disapproved of federal intervention in the case, with sizable majorities saying Congress was overstepping its bounds for political gain.
The public, by 63 percent to 28 percent, supported the removal of Schiavo's feeding tube, and by a 25-point margin opposed the law mandating federal review of her case.
A local court's decision to pull Schiavo's feeding tube was implemented later that March and Schiavo died of dehydration.
The recent admission by New York Yankees slugger Alex Rodriguez that he used steroids was the most recent in a series of reports that some of America's best-known and well-paid ballplayers had used performance-enhancing drugs.
No player, however, embodied America's realization that its athletic heroes were using steroids, like Barry Bonds, 44.
Since 2003, Bonds, who holds Major League Baseball's all-time record for most career home runs, has been under investigation for alleged involvement in a steroids ring conducted out of the Bay Area Laboratory Cooperative.
He was indicted in 2007 for alleged perjury and obstruction of justice for the testimony he gave before a grand jury.
Bonds testified in 2003 that he took two substances, which he referred to as the "cream" and the "clear," according to the San Francisco Chronicle, which obtained transcripts of Bonds' testimony. Bonds said he believed the substances were flaxseed oil and balm.
Bonds broke Hank Aaron's career home-run record in 2007 to considerable skepticism in the stands and sparked a debate about whether his record should be recorded with an asterisk given the taint of steroids use.
In 2005, in the wake of the BALCO and Bonds scandals, Major League Baseball instituted a steroids policy that included mandatory drug testing and suspensions for players testing positive for performance-enhancing drugs.