Wet masking tape is an especially stubborn material to remove.
Tearing it off in its sodden state leaves patches of soggy paper.
Yesterday, as afternoon showers came down on Hong Kong, these tiny patches were left all over the walls of the city's Central Government Complex.
A day earlier, as Hong Kong marked the 22nd anniversary of its return to Chinese rule, students protesting against a controversial extradition Bill had plastered the walls with banners, Post-its and posters stating their demands.
A group of them later trashed and vandalised the Legislative Council (LegCo) building in Admiralty after forcing their way in.
As an uneasy calm settled over the city, cleaners fanned out to clean up after the chaos of the night before. One 71-year-old cleaner told The Straits Times that he and 15 others started work at the government offices at about 6.30am.
"There were posters, placards, umbrellas, bottles and metal barricades all over the place, it was terrible," said the cleaner, who did not want to be identified.
It took the cleaners eight hours to clear the rubbish, scrape the posters off the walls and stack the dented barricades in piles on the sidewalk.
"These young people really made a mess of the place. How do you sit down and talk if you are like this?" said the elderly man.
Opposition against the controversial extradition Bill - which, if passed, will allow suspects in the territory to be sent to other jurisdictions, including mainland China - has been brewing for weeks.
Despite the Hong Kong government's claim that safeguards to ensure proper use of the law are in place, many Hong Kongers fear that it could be used to target political critics of Beijing.
The Bill triggered massive street protests last month, forcing Hong Kong Chief Executive Carrie Lam to suspend its passage. But the protesters are demanding that the Bill be withdrawn.
On Monday, hundreds of student protesters, some of them using a metal cart as a makeshift battering ram, broke through glass walls to gain access to the chamber.
They cleared out only when police used tear gas to disperse them in the early hours yesterday.
Hours later, as the financial hub got back to work, roads were unblocked, and police officers stood guard around the LegCo building.
But inside the darkened halls, black graffiti spray-painted by the students remained on the walls.
Engineering student Hades Tuan, 22, visited the site of the protests yesterday afternoon and placed three white roses near the "Lennon Wall", which was plastered with colourful Post-it notes, some of them calling for greater democracy. It is at the staircase heading to the government complex.
Asked about the violence on Monday night, she said the students did not intend to be violent. "But they feel like if they did not escalate the situation, we would not be able to get the attention of the government."
But not everyone agreed with their methods. Taxi driver Seto Wong Fai, 65, put it bluntly: "These young people had nothing better to do."
Said another Hong Konger, Mr Bryan Choi, 27, who works in a recruitment company: "Emotionally, I agree with them, but I wouldn't have done what they did."