Two years ago, I met a Chinese student who was entering graduate school in the United States. I told her I had been in Beijing during "6/4," the Chinese shorthand for the massacre of June 4, 1989.
"What are you talking about?" she asked.
At first, I thought she might not have understood my Chinese, but it soon became clear that June 4 meant nothing to her. I probably shouldn't have been surprised.
In the 20 years since that day in 1989 when Chinese troops opened fire on unarmed civilians near Tiananmen Square, Chinese censors have managed to erase all mention of that tragedy from the country's textbooks and state-run media.
But for me, Tiananmen is impossible to forget. As Beijing bureau chief for the Washington Post, I covered the student demonstrations that began in mid-April, tried to track a murky power struggle among top Chinese leaders, and managed a small team of young, Chinese-speaking American reporters.
What I remember best was the sudden openness of many Beijing citizens of all professions. They were inspired by throngs of students calling for political reform, media freedom, and an end to "official profiteering."
People I believed to be Communist Party supporters were suddenly telling me what they really thought. Some who had been silent in the past even debated politics on street corners.
In early May, Chinese journalists petitioned for the right to report openly on the Tiananmen protests, which on May 17 swelled to more than a million people marching in the capital. Journalists from all the leading Chinese newspapers, including the People's Daily, the mouthpiece of the Communist Party, joined in. Their slogan was "Don't force us to lie."
For a brief period, Chinese journalists were allowed to report objectively on the student protests. But this press freedom was short-lived, ending on May 20 with the imposition of martial law and the entry of the People's Liberation Army into Beijing.
At first, Beijing residents manning makeshift barriers blocked the troops. But late on the evening of June 3, tanks, armored personnel carriers, and soldiers firing automatic weapons broke through to the square.
The death toll quickly became a taboo subject for Chinese media. Chinese doctors and nurses who had openly sided with students on the square, and who had allowed reporters into operating rooms to view the wounded, came under pressure to conceal casualty figures.
One brave doctor at a hospital not far from Tiananmen Square led me and a colleague to a makeshift morgue, where we saw some 20 bullet-riddled bodies laid out on a cement floor. I later learned that the doctor was "disciplined" for allowing us to view that scene.
A Chinese journalist I considered a friend tried to convince me that government estimates of fewer than 300 killed were correct, and that these included a large number of military and police casualties. I later learned from colleagues of his that this journalist was working for state security.
After I compared notes with others, my guess was that the actual death toll was at least 700, and that most of those killed were ordinary Beijing residents.
It's almost incredible that the Chinese government has succeeded for so long in covering up a tragedy of this magnitude.
But for those who closely monitor the continued repression of civil liberties in China - and the government's stranglehold on news deemed "sensitive" - it's not surprising. Chinese authorities continue to intimidate reporters, block Web sites, and jam the broadcasts of outside news organizations. China is the world's leading jailer of journalists and cyber-dissidents.
Chinese youths are among the most Web-savvy in the world. But Chinese search engines, chat and blog applications, and Internet service providers are equipped with filters that block out certain keywords on a blacklist that is continually updated.
China's censorship is multipronged - sometimes heavy-handed and sometimes sophisticated, allowing debate on some issues and shutting it down on others, such as Tiananmen. Censors hold online service providers and Internet cafe owners responsible for the content that users read and post. A small blogging service will usually err on the side of caution rather than lose its license because of a debate about June 4.
Lines that cannot be crossed shift from time to time, leaving citizens uncertain and therefore prone to self-censorship.
The good news is that the blackout isn't complete. We know from Radio Free Asia's call-in shows that some younger Chinese know just enough about Tiananmen to want to learn more.
I work with several Chinese broadcasters who were students in Beijing on June 4. Many of them saw more than I did. And they are here to remind me - and many Chinese - of a history we should never forget.