High explosive powder is shaken into the nose cone of an improvised missile through a funnel fashioned from a mineral water bottle.
Then along comes a man with a long bolt. He shoves it down into the powder and starts whacking it with a steel-headed hammer.
One spark, a drift of cigarette ash, and the detonation of this arms factory would be heard and seen for many, many miles.
We agreed with our hosts, Syrian rebels with no connection to al Qaeda-linked groups, that we would not reveal the location of this installation. The reason was obvious.
For more than two years the rebels fighting Bashar al Assad had been begging the outside world for help.