A classic demonstration of a redox reaction.
Methylene blue is blue when oxidised but its reduced form is colourless. Glucose, in alkaline conditions, is a reducing agent. A bottle or flask is filled 2/3 or so full of a methylene blue – glucose – sodium hydroxide solution and the top sealed.
In a few minutes the solution goes clear. If it is shaken, however, oxygen from the airspace in the bottle gets into the solution and re-oxidised the dye to its blue form. on standing, it goes colourless again. And so on.
Blue Bottle – Risk Assessment