My nephew asked my how the water can burn in Cars 2 (see this clip, about a minute in) and I thought I’d share my answer here for any other children that have the same question.
Just after the spy car plunges into the water and is ‘shot’ by the missile (after which the fire appears on the surface of the water), you’ll notice some black/brown liquid trailing out of said spy car. That liquid is oil or petrol, both of which are flammable and so can easily burn if set on fire – by a missile exploding, for instance. They don’t mix with water though and they’re lighter than water. Because of this, the oil/petrol floats to the surface of the water and when the missile explodes it is this oil/petrol that catches fire. Because the oil/petrol is on the surface of the water, the fire burns on the surface too.
So, it’s not actually the water that’s burning.
If you, or a child in your life, has any other such sciencey questions, drop me a line and I’ll do my best to provide an answer.