Search behaviors?

[Jack Park]

I have several messages with "hello" in one form or another, e.g. Hello 
world, and hello there

When I type hello into the search bar, it brings up just the message 
with Hello world.

When I type world in the search bar, it brings up a message with 
"worlds" in it, but not the one with Hello world

What do we know about the stability and reliability of search? Are there 
specific heuristics, a command line to clear it and cause it to re-index?

Thanks in advance
Jack