What's a tweet, between friends? The law says sometimes it's a threat.
One man thought he was just bantering with his pals when he joked about blowing an airport sky-high. Another was reacting to a radio phone-in when he mused about stoning a journalist to death.
Because they made their throwaway comments on Twitter, both are in legal trouble.
Their cases have outraged civil libertarians and inflamed the debate about the limits of free speech in a Web 2.0 world. The Internet increasingly makes private jokes, tastes and opinions available for public consumption, blurring the line between public and private in a way that has left the law struggling to keep up.
"I think people don't have any idea of the potential legal ramifications of things they post on the Internet," said Gregor Pryor, a digital media lawyer at Reed Smith in London. "Anything you post on Twitter can come back and haunt you."
Paul Chambers found that out with a vengeance. The 27-year-old trainee accountant was convicted and fined after tweeting in January that he'd blow up Robin Hood Airport in northern England if his flight was delayed.
Chambers _ who lost his job and faces several thousand pounds (dollars) in legal costs _ said Monday that he has instructed his lawyers to take his case to the High Court, setting the stage for a major test of free speech online.
"Probably to the detriment of my mental well-being, I am appealing the decision as best I can," Chambers tweeted Monday.
Chambers is already an online cause celebre. After he lost an appeal earlier this month, thousands of Twitter users repeated his offending message _ "Robin Hood Airport is closed. You've got a week ... otherwise I'm blowing the airport sky high!" They added the tag "I Am Spartacus" _ a reference to the 1960 movie epic in which the titular hero's fellow rebels all assume his identity in a gesture of solidarity.
To many Twittizens, the outrage is obvious _ Chambers was no threat to anyone, just a frustrated traveler blowing off steam.
"It's worrying," said Evan Harris, a former British lawmaker and free-speech campaigner. "The judgment seemed to misunderstand that something said across Twitter was not a serious threat. This is not the mode of choice for any suicidal jihadist."
Twitter, he said, "is like chat in a pub."
"There is sarcasm in the pub," he said. "There is sarcasm on Twitter, which is understood by everyone on Twitter _ but not by that judge."
But others argue that it's not so simple.
The judge who rejected Chambers' appeal, Jacqueline Davies, said that "in the context of the times in which we live," with an ever-present threat from terrorism, Chambers' message was "obviously menacing."
Another ill-fated tweeter has received less sympathy than Chambers. Gareth Compton, a Conservative councilor in the English city of Birmingham, was arrested this month on suspicion of sending an "offensive or indecent message" after tweeting an invitation for a journalist to be stoned to death _ a comment he insists was a joke.
The subject of his tweet, newspaper columnist Yasmin Alibhai-Brown, reported him to police. He was arrested and questioned, but has not been charged. He later released an apology for his "ill-conceived attempt at humor."
Sympathy for Compton was relatively muted. Liberal Twitterites may have felt less comfortable supporting a Tory politician who'd attacked a Muslim woman.
But Harris said Compton's arrest is equally unfair. He said Compton's message _ "Can someone please stone Yasmin Alibhai-Brown to death? I shan't tell Amnesty if you don't" _ was "obviously not a serious menace."
Legal experts agree that the law is not keeping up with technology and the ways it is changing communication. Chambers was convicted of sending menacing electronic communication, under legislation originally introduced to protect telephone operators from indecent calls.
Many people have learned that unguarded online comments can be embarrassing. Just ask Peter Broadbent, the Church of England Bishop of Willesden, who apologized Monday for greeting news of the engagement of Prince William and Kate Middleton with a tweet about taking a "republican day trip to France."
Broadbent apologized and said he'd been unwise to get into a debate "on a semi-public Internet forum," but his boss, the Bishop of London, said Tuesday that he was being suspended from public duties "until further notice."
Around the world similar cases, though in different contexts, are testing the limits of what can be said online.
In China, where the Internet is restricted and Twitter is blocked, a woman was recently sentenced to a year in a labor camp for "disrupting social order" by retweeting a satirical message urging Chinese protesters to smash the Japan pavilion at the Shanghai Expo. Her supporters said the retweet was meant as satire.
In the United States _ where the First Amendment right to freedom of speech is seen as a beacon by British civil libertarians _ the National Labor Relations Board is challenging a case in which it claims an ambulance worker was fired for criticizing her boss on Facebook. The board's lawyer said such comments are "the same as talking at the water cooler," and so protected by law.
Pryor said such cases show that the legal balance between freedom and responsibility is still being worked out.
Julian Glover, an editorial writer with the Guardian newspaper, thinks it will be a while before things settle down.
The Internet, he wrote recently, is a "life-changing invention that will take time to develop civilized rules of its own" _ just as automobiles were followed by highways and then, after time and pileups, by speed limits.
"The Internet is nearing its speed-limit stage," Glover wrote. "We can't guess where this will end, only that the skirmishes have only just begun."