Why can’t they invent a car that runs off water, meaning the explosive power of hydrogen and oxygen?

If you mix hydrogen gas and oxygen gas together without the heat, isn’t it explosive? So if you had a car that could convert water back into these gases and then cause an explosion and channel the explosion into energy, why couldn’t you use the energy to run the car?

would the government pay for my patent if i invent a new way to make green energy?

i think i know a way to make reliable energy with out polluting the earth. i want to patent this idea and get it out there but i don’t really have the money, i am only 18.. would the government help me or something?

Engineers: why don’t you invent a device to convert a regular car into a green one?

Green cars are the best solution to keep our environment clean; but we, poor people, cannot afford to get one, and everybody knows that we are everywhere….