The only fuel that forms only water as a byproduct when burned is hydrogen.

Here's why:

* Chemical Reaction: Hydrogen (H2) reacts with oxygen (O2) to form water (H2O).

* Equation: 2H2 + O2 → 2H2O

This reaction releases energy, making hydrogen a potential fuel source. However, it's important to note that:

* Practicality: Storing and transporting hydrogen can be challenging due to its low density.

* Production: Producing hydrogen often requires energy, and the methods used can have environmental impacts.

While hydrogen is a clean-burning fuel, its widespread use faces significant challenges.