The zebiba used to be the mark of an elderly Muslim man, the fruit of a lifetime's devotion, but it is increasingly seen on the faces of young Egyptians.
Literally meaning "a raisin", the zebiba is a patch of hardened skin where the forehead touches the ground during Muslim prayers.
Some welcome the trend as a sign of devotion, others say it is ostentatious piety.
Worse still there are fears public displays of faith like the zebiba and the hijab, or headscarf, are spilling over into vigilantism.
Liberals or Christians who don't conform in the workplace or on the street say they are being harassed.
Gift from God
A practising Muslim's forehead is meant to touch the ground at least 34 times a day - in symbolic submission to God's will - which could add up to more than a million prostrations in a lifetime.
The relentless rise of political Islam over the past few decades has succeed in rolling back significant parts of Egypt's secular tradition But over the past few decades, as more and more Egyptians turned to religion, the zebiba began appearing among young men as the veil did among young women.
But not every Muslim gets one, and opinions vary as to where it comes from. It could something to do with skin-type, or created artificially, or come from particular kinds of matting. Others believe it is a gift from God.
Many young Egyptians I asked believe some kind of light will emanate from the prayer mark on their foreheads on the Day of Judgment, marking them out as truly devout.
One of Egypt's greatest living and most popular poets, Abdelrahman al-Abnoudi, has another explanation - in times of crises people turn either to drugs or to religion.
Egyptians have always been religious, he adds, but since being religious has also become fashionable, people now press their foreheads against the ground a little harder to acquire the appearance of a devout Muslim.
Dalia Ziada of the American Islamic Congress - an non-governmental organisation based in Cairo - says some men deliberately pray on straw mats, and rub their foreheads until they eventually develop the zebiba.
The increased public display of religious devotion is part of a wider phenomenon, affecting what women wear, and what people read or watch on their television screens.
In recent years the zebiba has become a pronounced mark on foreheadsThe relentless rise of political Islam over the past few decades has succeed in rolling back significant parts of Egypt's secular tradition.
For radical Islamist politicians, like Magdi Hussein, that is a move in the right direction, away from the Westernisation which started over two centuries ago with the French and British invasions.
He sees the the zebiba phenomenon in the context of government-inspired "darwasha", an atmosphere of unpolitical religious devotion which goes against the Islamists' self-professed aim of reforming society and fighting corruption and despotism.
But Mr Hussein refuses to acknowledge that the increased public display of piety has had any downsides.
Suddenly the girls - all of whom were veiled - surrounded the car and start banging on the windows
ShahinazEgyptian women and Liberals I spoke to tell a different story. A Coptic doctor, who did not want to be named, told me she had been spat upon in broad daylight for not wearing the veil.
A young Muslim engineer, Shahinaz, who refuses to cover her hair, said she has become scared of intimidation.
"I was driving home one evening and had to stop next to a girls' school. Suddenly the girls - all of whom were veiled - surrounded the car and start banging on the windows and screaming: 'Infidel! Apostate!' I was terrified."
Dr Sayyed al-Qimni, one of Egypt's best known liberal writers and historians, says society has been hijacked by a very conservative brand of religion, which he characterises as Saudi Wahhabi Islam.
"There are now 13,000 religious schools [in Egypt] that produce terrorists, like the Taliban madrassas in Pakistan. At religious schools they teach children that Muslims who do not pray should be killed."
There is no doubt that in one way the Islamists are winning their struggle to increase the role religion plays in social life and public debate in this country.
The question now for Egypt is what kind of Muslim society it is going to be - one that is at peace or at war with modern values.