As an estimated 2 million Hong Kongers stormed the city’s streets on Sunday, protesting a controversial extradition bill and demanding the resignation of Chief Executive Carrie Lam, police continued their assault on residents using tear gas, beanbag pellets, and rubber bullets. A week after first taking to the streets, the protesters have demonstrated remarkable resilience and organization in their response to the violence. One particularly scrappy display of defiance, images of which circulated widely on Twitter, shows protesters swarming a cartridge of tear gas that had been lobbed by police and swiftly diffusing it with a few bottles of water.

This mesmerizingly simple but crafty way of thwarting riot police sparked admiration and curiosity. Tear gas is one of the most dreaded crowd control weapons. Could it really be that simple to defuse the stuff and spare protesters from injury?

Actually, yes, but the size of the weapon that protesters are up against is crucial. Overall, tear gas chemistry is actually pretty simple. According to Sven Eric Jordt, an associate professor of anesthesiology at Duke University and expert in tear gas, the pure substance behind it is not a gas at all but a whitish powder at room temperature. It has to be heated in order to cause its notoriously painful effects—coughing, wheezing, temporary blindness. That happens through a pyrotechnic charge in the tear gas cartridge, which creates diesel fireworks that cause particles to be dispersed into the environment as an aerosol gas.

Without the fiery charge, nothing would happen. That’s the main reason why water stymies the reaction—by killing the charge so that no vaporized particles can be released. On top of this, the tear-producing substance in the cartridge, called CS, is also sensitive to water, which means it’s basically inactivated when protesters start pouring. That’s why water is also used to treat people who have already been victim to tear gas; it stops the whole painful reaction.

“What you see in this video is that the smoke coming out of the cartridge is extinguished and [the protesters] basically just throw it away,” said Jordt.

Impressive how thoughtful and organised these protestors are. Bottles of water are left along pavements and other strategic areas so people can douse themselves, and put out teargas grenades when police move in #HongKong #AntiELAB #反送中 pic.twitter.com/Oq1FQHaeXc — Lianain Films (@LianainFilms) June 12, 2019

But if it’s that easy to put out tear gas, why don’t protesters everywhere do this all the time?

While dousing canisters with H 2 O, as seen in the clip, really does work, it should be noted that the cartridge that protesters watered down in the video was unusually small. A few water bottles might not be enough to defuse anything larger than that. Police usually have a lot of crowd control weapons at their disposal, especially at protests as massive as the ones in Hong Kong, so it’s likely that this amount of water won’t be sufficient if officers bring out the big guns.

Jordt says he hasn’t seen the tactic used very frequently. That’s probably due to the difficulty of organizing a water blasting for every canister—the Hong Kong marchers were exceptionally well-organized—as well as the possibility of police retaliation at protesters for using this tactic.

There is video of an even more extreme method that was tested during the 2013 Gezi Park protests in Turkey. There, protesters picked up large canisters of tear gas and shoved them in jugs of water, which were then quickly sealed. In the video, this technique appears to work, but it runs a serious risk of causing an explosion.

Now that images of the Hong Kong method have circulated widely, there is a real possibility that it will become a more mainstream way of countering police violence in the future.