It does not work with "where", "who","when" and "how" either…
There are a bunch of common words that it does not index. All of these are examples of that. They show up so commonly that indexing them becomes pointless -- more noise than signal.
Can't search for the word 'what'. Thread i was searching for was "Another what did you do to your Vespa today thread"
This is fairly standard in search indexing.