It's not a stupid question at all - it's an area that Google has worked on for the entire time of their existence. But the answer isn't simple at all - the best we can see is "it depends". It depends on how strong the semantic "identity" is between the words, whether either one has other meanings (and usually at least one does) and so on.

If you want a glimpse of what Google sees as the strongest semantic identities between words, use the tilde operator [~] before a keyword. When there is a strong semantic identity, the secondary word will also appear in bold. For example, I just did a search on ~tax, and in bold I see IRS, taxes, taxation, & property.

But most of the time Google doesn't treat the different meanings as exactly the same, but at times they will give a search result that includes a synonym or semantically related word. There's a lot of relative weighting and user data all rolled into how they handle any given word or variation or synonym. Exact text match still gets the most weight, almost all of the time.

i don't believe there is ever a case where 2 words have exactly the same meaning, although as tedster says they can be synonyms.

in your case (i realise you just used it as an example) they are not the same - H2O is a chemical formula and it represents pure 100% water, whereas water in real life is never pure unless it has been specially treated - i don't believe the words should be used as exactly equivalent.

@ topr8 actually there are plenty of abbreviations that mean exactly the same for instance PC and personal computer.

@ tedster thanks last sentence is what I speculate too.

PC isn't a word it's an acronym, there's a symantic difference

and in the case of acronyms and abbreviations you should use the appropriate tag - eg <abbr> or <acronym>

PC may not technically be a word but it is a common keyword entered into search engines.

There is also another situation where two words can mean the same thing - typos. One user searching for "symantic" & another searching for "semantic" are both looking for the same thing.

@ topr8: your right it is an acronym, sorry i'm lost where would you use the tags do you mean in net language?

In the case of acronym would anyone say Google gives them equal value? I would say exact keyword would still get priority regardless of acronym.

topr8 actually there are plenty of abbreviations that mean exactly the same for instance PC and personal computer.

PC may not technically be a word but it is a common keyword entered into search engines.

PC could also be considered to be an acronym for "politically correct"

Many letter combinations that make acronyms are referring to totally different word groups and thus have totally different meanings ( and that is without taking into account different cultures which use the same language and thus generate identical acronyms for different things )..nor different languages which may generate the same or similar letter patterns as English..but create acronyms that have no relation to with the meaning of the English acronyms..

Thus topr8 is correct ..even if you restrict your search to one single language you will get many identical acronyms ..all with different meanings.

A funny example along these lines was a search I made last year for "bread". The number three position for a short time was for "Rolls Royce".

At first I thought it was because "bread" can mean "money" and to buy a Rolls takes a lot of bread.

But them I noticed that only the word "Rolls" was shown in bold, and not "Royce." That clue gave me the answer - "bread" and "rolls" have a strong semantic relationship, and the tilde operator confirmed it.