A gas explosion in a dessert glass produces a whole range of colours from red to turquoise to bright blue, with a lot of turbulence. For this effect, the glass is first filled with a mixture of butane (lighter gas) and air, after which it is ignited with a high voltage pulse (spark). Since butane is heavier than air, it remains in the glass. Various effects can be achieved by adjusting the gas/air mixing ratio. The electrodes and the spark for ignition have been photoshopped out of the picture as it would otherwise be distracting.