Question

Why is the only byproduct of burning hydrogen water?

Answer 1

The word "hydrogen" is Greek for "water-former".

If fuels are burned, the elements in the fuel combine with oxygen to form oxides. In hydrogen only hydrogen-oxide (=water) can be formed, as no other elements are present.

If hydrogen `H_2` is burned, it connects to oxygen `O_2` in a ratio of 2:1 as follows:

`2H_2+O_2->2H_2O` which is water.

Extra:
Most fuels contain other elements, like carbon (`C`)
Carbon combines with oxygen to carbondioxide (`CO_2`)

Example : natural gas or methane (`CH_4`) burns like:

`CH_4+2O_2->CO_2+2H_2O`

So the carbon-atom burns to carbondioxide, and the hydrogen burns to water (vapour).