How heavy is a flame?

Just one of those intriguing questions I was asked once. Have a long hard think and later I'll post what I discovered down the pub... so it must be right.

Meanwhile, what reminded me of the question was this pic I took at a German friend's recent Feuerzangebowle party. In a nutshell, it's mulled wine which has a sugar cone held above the Feuerzangebowle (bowl) on a Feuerzange (fire tong), soaked in rum and set alight.

Viva Europe!