Are you convinced you have freedom of speech at work? Think again.

The First Amendment, experts are quick to point out, doesn't extend nearly as far as you might think.

The U.S. Constitution's free-speech protections mean the government can't prevent you from expressing your opinion.

By contrast, with limited exceptions, there's nothing to keep your employer from firing you or taking other disciplinary action for what you say at the water cooler, in a company newsletter, or even on your own personal blog.

"A lot of people think they're protected by the First Amendment in cases where they're not," says David Williams, a partner at law firm Morris James LLP in Wilmington, Del., and a past chairman of the employment and labor law section of the Delaware State Bar Association.

Union collective bargaining agreements and individual employment contracts generally say a worker can be fired only for what is known as "just cause."

But 92 percent of private-sector workers aren't covered by collective bargaining, and employment contracts are usually limited to high-level executives.

For most private-sector employees, "if you say something critical of your boss, you can lose your job in a heartbeat, and there's nothing you can do about it," says Lewis Maltby, president of the National Workrights Institute, a workers' rights group based in Princeton, N.J.

In the age of the Internet, a worker's comments, or actions captured in an online video, can spread quickly across the country, if not the world.

"You become a public-relations embarrassment, so you get fired," says Matt Finkin, a University of Illinois law professor and author of the book, "Privacy in Employment Law."

Just five states — California, Colorado, Montana, New York and North Dakota — have laws that protect an employee's right to free speech, and there has been no push in Congress for passage of a national law, Maltby says.

"The general rule, in the private sector, is that employees are terminable at will," says Martin Malin, a professor of law and director of the Institute for Law and the Workplace at Chicago-Kent College of Law.

Government workers have slightly broader First Amendment protections, Malin says, but adds that even there the law generally protects them only for comments made as a private citizen, not in the course of them performing their job duties.

Workplace experts say a growing number of employees are getting fired or disciplined for their comments, including remarks posted on personal blogs or on social networking Web sites like MySpace or Facebook.

Blogging and social networking sites are growing in popularity. Eight percent of Americans say they blog, and 16 percent use sites like MySpace and Facebook, according to the Pew Internet & American Life Project.

At the same time, workers are lulled into mistakenly thinking they can say things on the Internet they'd never say in person, and that the boss won't find out about it.

"They forget about Google," Maltby says, "You can find anyone's blog in 10 seconds with Google."

Companies may have legitimate legal or business reasons for checking on what employees post on the Internet, including whether they are disclosing trade secrets or other proprietary information. They also are using the Web to learn more about prospective hires.

A September 2006 survey by the Society for Human Resource Management found that 9 percent of companies said they use search engines like Google and Yahoo to do background checks on job candidates or to pre-screen them.

"You may otherwise be a capable employee, but you're not the only applicant," Maltby says. "You're basically getting married to a stranger, and if they pick up any indication, most employers will play it safe. It's general risk aversion. Nobody wants to hire a nut case."