LVIV, UKRAINE (NYTIMES) - The instructor held his hand above his head, revealing a tiny, wavy green piece of plastic nestled in his palm.
"See this? Some civilians told me they saw these on either side of the humanitarian corridor they were walking down. They looked to them like leaves," said Mr Serhii Romaniuk, the teacher, explaining that the green plastic "leaf" in his hand was actually a deactivated land mine. "It was called a safe corridor, but that was a lie. It was mined on either side."
His voice boomed across the dark subterranean room - a school cellar arranged like a military bunker, plastered with posters about different weapons and tires for exercise drills.
Listening attentively were a handful of muscular young volunteers from a local defence force, who crammed themselves into the circle of wooden school desks. Sitting among them also were civilians: young boys, women and older men.
Basic military preparation courses offered in cities like Lviv were developed with local volunteer defence forces in mind. But now teachers like Mr Romaniuk are opening their doors to civilians, in particular those who have fled fierce battles in the east to the relative safety of western Ukraine.
The civilians at Romaniuk's course live inside the classrooms of the school above. And they have one goal in mind: "to go home," said 13-year-old Nikita, from the Donetsk region, a key target of Russia's territorial aims.
"But when I go back home, I need to know how to react around weapons or land mines that I might come across. I'm learning all kinds of things here. I didn't know, for example, that mines could be connected to each other and timed," he said.
The war has displaced more than 12 million Ukrainians, more than half of whom fled to safer regions in Ukraine's west. But now tens of thousands of civilians are returning to the places where their army has beaten Russian forces into a retreat. And some of those places could be literal minefields.
According to the UN, Ukrainian forces have removed nearly 80,000 mines and explosive devices since the full-scale invasion. But it will still take years to remove all of the mines in Ukraine, it said.
Mines have a debilitating affect on civilian lives because even just the possibility they are present can be paralysing. Many farmers in recaptured territories have found themselves unable to sow their fields after the Army warned them it suspected mines had been planted but said that it was unable to sweep the areas yet.
Mr Romaniuk, a white-haired military veteran, held up a heavy, circular tin that he said was an anti-tank mine - the type most often found in farmers' fields.
"If you know about them, how to spot them, then you're armed at least with knowledge of what to do," he said.
His course is not just about land mines: Mr Romaniuk also runs basic military drills. And he gives lectures on how to determine the types of missiles being used, the shock waves they create and what to do when a bomb hits.
It is his civilian students for whom that knowledge is no longer theoretical. The local volunteers are from Lviv, barely damaged by the war, and have never experienced the type of shelling that the civilians they share the classroom with have faced.
Mr Ruslan, a 44-year-old from the embattled city of Sievierodonetsk who requested his last name be withheld because he fears repercussions for family in Moscow and Russia-held areas, knows what it is like to be smashed into a wall from a blast wave. He has survived siege and missile strikes and escaped his home under mortar fire.
Listening to the lecture brought back painful memories. "It's really hard. But what choice do I have?" he asked.
"We have a terrible war going on, and that affects everyone, whether you're a grandmother or a child," he said. "We have to try and save ourselves."