Odourless, tasteless, and absorbed almost instantly on contact in gas or liquid form, Sarin is perhaps the most feared of chemical weapons.
Because it is heavier than air and can saturate material including clothes, it can linger for several hours, especially in low ground.
How does it work?
But it doesn’t directly poison its victims – instead, it turns the human nervous system against its owner.
The real killer is something called acetylcholine – a neurotransmitter that our nerves send out to instruct muscles to contract or relax.
It is a process that takes milliseconds, and keeps involuntary organs like heart, lungs, and intestines working day in, day out.
Sarin's deadly trick is to disable the enzyme that normally breaks down the neurotransmitter - leading to rapid build up of surplus messages ordering muscles to act.
Bombarded by intense and repetitive instructions, the victim's muscles try to comply - with undignified, painful, and often fatal consequences.
The immediate symptoms of Sarin exposure - a runny nose, crying, involuntary urination and defecation - are all a result of muscles involved being overloaded with orders to move.
It is not always deadly. But in the absence of antidotes, convulsions, paralysis, and death can follow in minutes.
Most victims, like the children in Idlib, suffocate after losing control of the muscles that control breathing.
What is it for?
Chillingly, it is not simply the weapon of deranged dictators.
During the Cold War, both the Soviet-bloc and Western militaries maintained stocks of deadly chemicals and tactical doctrines about how and when to use them the battlefield.
“Sarin quickly causes casualties over a defined area,” said John Gilbert, Senior Science Fellow at the US-based Center for Arms Control and Non Proliferation.
“And because it dissipates quite quickly, usually within a few hours, you can then send in your own troops to take that area,” he added.
By contrast VX, the nerve agent used to murder Kim Jong Nam, the brother of North Korean leader Kim Jong Un’s brother, in Kuala Lumpur airport in February can hang around in lethal concentrations for days after it has been released, and is viewed by militaries as an “area denial” weapon that can make airports or other areas unusable.
During the Syrian civil war, Bashar al-Assad’s forces have generally used chemical weapons – mostly chlorine gas – to spread confusion in areas where regime troops are on the offensive, said Igor Sutyagin of the Royal United Services Institute.
"The other tactic is like a sniper, lure rescuers to the scene and then hit them. It is a way of concentrating people to the target," he said.
Where is it made?
Sarin is not an easy thing to produce, and if mishandled the process can be extremely dangerous.
But nor is the process impossible to replicate. In 1995, the Aum Shinrikyo suicide cult produced enough in its own lab for a deadly attack on the Tokyo metro.
Syria may have retained some of the stocks it was meant to give up. But it is also conceivable that the regime simply made more, using improvised facilities and ingredients it had not prepared.
Loading it into bombs or rockets to deliver it would be one of the riskiest parts of the operation, but would not be beyond intelligent amateurs with a good grasp of chemistry and who were extremely careful, experts say.
At it's crudest, liquid Sarin could conceivably be loaded in a barrel and thrown out of a helicopter. "You'd just have to consider the crew to be very careful or very expendable. If it leaked it would kill everyone on board," said Mr Gilbert.
Why isn't it banned?
Growing disgust amongst the public, the difficulties of safely storing and maintaining stocks, and the unpredictability of the weapon – a sudden change in wind direction could see the poison blow onto one’s own troops – contributed to the 1993 Chemical Weapons Convention, under which almost every country on the planet has committed to destroy its stocks.
Syria was effectively forced to accede to the convention after the Ghouta Sarin attacks in 2013 – when the United States and Russia compelled Assad to give up his stockpiles.
Officially, the destruction of the Syria’s chemical stockpiles under international supervision was completed in 2014.
It now seems like some of those stocks were overlooked, or deliberately hidden, or replaced by new supplies.