Firstly, the definition from Wikipedia: Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
For all you bad cooks out there, the reason you can’t burn water when you’re cooking is because water is already fully oxidized. Water is also often one of those reaction products the definition talks about.
I other words, you can’t burn water because it’s already burnt.