The thoughts beneath his large black hat were, like those of most males weathering a testosterone-flooded adolescence, turning to women. His mistake, as an ultra-Orthodox Jew living under the eye of the rabbis, was to talk about it.
Punishment was swift and unpleasant. He was banned from conversing with any other students in his Jerusalem yeshiva, or religious study centre. His father was under orders to escort him home. A personal tutor was organised to ensure that his ugly, lustful impulses did not rear up anew. Being an outcast among your own kind is, he recalls, "worse than serving 10 years".
Four years later, Noam (he has asked for his surname to be withheld) decided enough was enough. Despite the risk of alienating himself from his family, including six siblings, he left.
In doing so, he became an addition to a lengthening list of young people who have escaped over the walls of Israel's closeted but powerful haredi world and entered secular life. Suddenly, that number is now sharply rising; Hilel, an Israeli organisation set up to assist those leaving ultra-Orthodoxy, says that its caseload this year has more than doubled. By October, the figure had swelled to 237, compared with 107 throughout 1998.
"We have no explanation," said Ami Dolev, a Berkeley- educated academic and liberal who heads Hilel. One theory is that it reflects the increasing extremism within some elements of the haredim, a reaction to the flood into Israel of nearly a million secular Jews from the former Soviet Union over the past decade. Another is that some ultra- Orthodox groups have embraced the mobile phone and the internet, allowing their youths unprecedented access to the outside world.
Noam acted just in time. Five months earlier, after a childhood in which he was segregated from the opposite sex he was allocated a wife in an arranged marriage. Had the couple submitted to the communal pressure to have children, he would probably still be living a life in which every decision was made for him - from his clothes (black double-breasted suit and hat, white shirt) to his kosher diet, to the timing and manner of his sex life. It was not just a matter of living in a community, he recalls. "It was like a country. It was everything." So confined was his life that he did not know how to find Jerusalem's famous Jaffa Street, although it was 300 yards from his home.
He wanted more than that. He wanted independence and choice. Television, radio and newspapers are usually banned by the haredim. He broke the rules by secretly buying a television set, which he set up in his three-roomed flat in Jerusalem's ultra-Orthodox Mea She'arim quarter, whose walls carry notices warning women to "dress modestly" and whose residents have been known to hurl stones at those who do not. Smuggled in within a microwave box, the TV was kept in a cupboard when he wasn't watching it. The first film he ever saw starred James Cagney; he still recalls his astonishment at the discovery that there was such a being as an actor.
Drawn toward the wider world beyond, Noam rang the Hilel organisation's hotline from a phone box. After meeting one of their representatives, he packed his belongings, had a final conversation with his wife (without revealing his plans) and walked out. On the same day he cut off his peyot, the ringlets worn by ultra-Orthodox Jews, and his beard. "That made it impossible to go back," he said, sitting in a Jerusalem cafe clad in the symbols of secular life; jeans and a sweatshirt.
Life on the outside is not always easy. Noam, aged 26, is now studying ancient Greek and history at university, paying his way by working part- time as a security guard. But the typical 18-year-old leaving ultra-Orthodoxy has the general education of a secular nine-year-old. He - or, more rarely, she - has little knowledge of the opposite sex.
"They are completely different, but society doesn't realise it," said Dr Dolev, "so they have tremendous emotional difficulties. They don't know the basic codes of behaviour." Noam has an eloquent way of putting this: "The secular world thinks every guy with a beard is a rabbi; the religious community thinks every girl on the outside is a hooker." The departees also carry a burden of guilt, knowing that they brought shame on their families, undermining their status in the synagogue and damaging their sister's marriage prospects; Noam still sees his parents, but relations are fraught. They will, he says, never understand why he left.
Hilel sets out to ease these problems, by providing safe shelter, education and advice to new arrivals to the secular world. It is strictly apolitical and makes a point of never trying to lure ultra-Orthodox youths away from the fold.
But Dr Dolev makes no secret of his dislike of haredi society (a catch- all term covering at least 96 separate groups and more than 300,000 people), which he compares with a "chicken hatchery", comprising people who cannot tolerate independent thought. "They are told when to wash, what to eat and when to pray ... the rabbi tells them how to vote."
His views are shared by many among Israel's secular majority, who see the rift with the religious world as the country's largest problem. The secular resent being dictated to by the rabbis over issues such as kosher food and marriage, both of which the rabbinate control. Haredi society is seen as a threat to democratic pluralism as well as to the economy. To secular annoyance, the ultra-Orthodox rarely serve in the army, although they often attract hefty welfare payments because of their large families and low incomes. And the haredim look on the rest of Israel with deepening distaste as more and more non-religious Jews crowd in.
Hilel has good reason to be hostile. Its volunteers have found that helping ultra-Orthodox youths can be a sinister business. Earlier this year, listening devices were discovered in its offices in Tel Aviv and Jerusalem. According to Dr Dolev, the premises were also broken into and the computers tampered with. One youth who made the mistake of telling a friend that he was planning to escape was beaten up. Another was, Hilel says, offered a bribe to provide information to the rabbis. And haredi opponents have cunningly launched a telephone hotline with exactly the same name, in the hope of heading off escapees.
Will the gulf between the Israel's secular and ultra-religious identities ever narrow? Noam doesn't think so, even though some haredi groups recognise that they must selectively modernise. "There will never be a bridge. In fact, they are growing farther apart." Nor does Dr Dolev. Asked whether the rise in the numbers of those departing ultra-Orthodoxy means haredi communities are doomed to collapse, he had a blunt reply: "Not in our lifetime."