The air around Cairo's Tahrir Square is thick with tear gas, so much so that just upon entering the vicinity of the square eyes begin to water, and the sneezing begins - as though everyone in the area has the same cold.
But the reality is more insidious than the average cold - it lingers in the air, stinging the eyes and burning the lungs.
Despite the dense cloud hovering in the darkness, thousands of people remain in the square.
Though the crowds appeared to be thinning out earlier in the evening, scores of protesters began streaming back into the square several hours later, determined to help occupy Tahrir.
The crowds frequently break out in chants - a few voices are soon joined by a chorus of hundreds who shout rhyming couplets calling for the downfall of the military, and with it, Egyptian Field Marshal Mohamed Hussein Tantawi.
Across the square, hundreds of other protesters chime in with their own chants, yelling "hurriya, hurriya" or "freedom, freedom".
Remaining through the night
At 10pm, the night feels much like it did 24 hours ago, when intense clashes erupted around Tahrir.
The atmosphere is tense, with the almost tangible expectation of something ominous to come and sirens wailing in the distance.
Protesters bang pieces of metal against the sides of buildings and break pieces of pavement to form large rocks - all in preparation for something. What that something is, though, no one is sure.
One protester, equipped with the requisite bandana around his neck (perfect proximity for use when a sudden wind sends a cloud of tear gas wafting his way) asks for the time.
He requests to remain nameless, but says he is prepared to stay in the square all night. He is not alone, and well after midnight, and into the early hours of Monday, thousands remain in and around Tahrir.
When asked, several say they don't know how much longer the stand-off will continue, but that they will continue their protest for as long as it takes.
However, with the violence occuring just days ahead of the start of the country's first post-Mubarak parliamentary elections, uncertainty reigns supreme.
Among the crowd in Tahrir on Sunday is Stephen McInerney of the Project on Middle East Democracy in Washington.
He is in Cairo partly to serve as an international witness to the polls, but says it is "quite possible that they'll be delayed" due to the protests.
Most importantly, he says, is that the protests mark the latest evidence that the ruling military "has not been steering the country in the direction of democracy".
"[They're] not meeting the demands of the people," he says, "so as a result we're seeing people back on the streets."