Opposition groups in Syria are claiming anew that the Syrian government used chemical weapons against them. This time, they say, the attack killed over a thousand people, including children. The Assad regime has denied the claims, and United Nations inspectors in the country have not been able to independently confirm or deny them.
It's not known what type of chemical weapon might have been used in the attack. But how do different kinds work and what makes them so terrifying?
PHOTOS: Deadliest Poisons Known to Man
A handful of chemicals rank among the most feared as potential weapons, according to HowStuffWorks.
On the less-threatening end is sarin, a clear and tasteless manmade liquid, which the Syrian government is rumored to have, according to the BBC.
Sarin affects communication between nerve cells by interfering with an enzyme called cholinesterase, which works to flush out a message-carrying molecule called acetylcholine. By knocking out the off-switch normally provided by cholinesterase, HowStuffWorks explains, sarin causes uncontrollable contractions of the muscles, including the diaphragm. Suffocation can follow.