Try it yourself. Type "want" in the top left box and press enter. I want it corrected!
Sometimes I remember the phrasing of the question I am searching and it doesn't contains any keyword, so I have to go to Google to search it.
They're called stop words.
One of our major performance optimizations for the “related questions” query is removing the top 10,000 most common English dictionary words (as determined by Google search) before submitting the query to the SQL Server 2008 full text engine. It’s shocking how little is left of most posts once you remove the top 10k English dictionary words. This helps limit and narrow the returned results, which makes the query dramatically faster.
As of today, we do a better job of discarding stopwords
The search ignores common words and focuses on keywords.
You can see what keywords it's searching for by looking on the right side of the search results. You can also switch tabs and then check the search box.
I would expect it to ignore "common" words - most search engines do, perhaps accepting them if you force them (google, yahoo, ebay, amazon, etc).
Indexing common words slows the search down, and returns results you [probably] don't want.