Thinking About Knowing

I've been thinking a lot on the old advice of 'write what you know'. I used to be a real estate agent, so if that advice were taken to heart, all my writing would pertain to housing and buying houses. Then again, I live on a farm, maybe all my writing should be about farm animals. The thing is the real estate license was over 12 years ago. Yeah I knew a lot about it, but if I tried to write it now, I'd be completely lost. I'd have to do a lot of research. I think THAT'S where writer's really have to get their 'knowledge'. You can write about anything if you study it well enough.