"Gas detector" prop with 4 activation keys - it is decorated plywood box with a size of 60-15-40 cm. There are 4 indicators to see «level of gas» and 4 taps to control it. Also there is another decorated box with a size of 12-7-10 cm with places for four activation keys. Each key activates its own control tap.
Players have to set level of gas in «green» zone to open the lock.
When taps are opened, gas level increases, when closed - falls down.