Extinguishing candles with sulfur hexafluoride

It's pretty straight forward.

Combustion - like the fire on the tiny candles - needs oxygen. Oxygen is slightly heavier than air (because air is mostly nitrogen gas, and oxygen is heavier than nitrogen), but sulfur hexafluoride is way heavier than oxygen or nitrogen or air.

So, the sulfur hexafluoride gas displaces the air, leaving no oxygen for the combustion reaction to burn.

It would be just like filling the large box with water, but you just can't see the sulfur hexafluoride.

Of course, there are problems with freely dumping sulfur hexafluoride out.