Write What You Know: A List.

According to Mary, you should write what you know.  As a side note, whenever someone says the phrase "what you know" I hear Tom Cruise yelling "Don't tell me what I know and what I don't know.  I know the law."   Of course, I don't know the law, so it doesn't apply to me.  I would probably never write about the law.
What I could write about:

• the Navy
• being a sister
• having a sister
• having a brother
• pets
• parents
• parent's death
• old people
• babies
• young people
• nuclear power
• rocket science
• space
• school
• desperation
• dreams
• hopes
• small apartments
• large bedrooms
• rats
• bicycles
• ladders
• swimming
• gardening
• tractors
• tire swings
• softball
• anxiety
• drinking 
• sleepless nights
• alarm clocks
• breaking and entering
• house fires
• road trips
• hunting
• weapons
• computers
• moving vans
• diabetes
• cancer
• G Chat
• generosity
• deadlines
• food service
• coffee
• defense contracting
• appeasing others
• mothers
• motherfuckers
• douchebags 
• Canada
• the Midwest
• the Northeast
• the Northwest
• guitars
• disappointment
• horror
• disgust
• technical reports
• television
• 30 Rock
• the evolution of female characters in media
• comedy films
• heart break
• bulletized lists

That's just to name a few, and that was just off the top of my head.  And I have clearly taken Mary's point and flipped it on its head.  The sentiment of writing what you know is incredibly valid, and the problem with the sentiment is too many people take it at its face value.  That's how we end up with so many stories about struggling writers.  I think it's why Architect is such a popular occupation for a protagonist.  It walks the right line of being vague and specific.  The protagonist does something but it could be anything.
     Anyway, I digress.  Mary's right.  Write what you know.  Also, if you want to write about something and you don't know a lot about it, find out.  It's called research.  If you want to set your story in Houston but you don't know a lot about it, find out; read a book, check out the internet.  Then you'll know.  Then you can write about it.