So you know that creepy crawly miniature lobster creature that you can find in lakes and streams, usually under or near rocks? What would you call that (besides gross)...
A. Crawfish
B. Crayfish
C. Crawdad
D. You've never heard of this crustacean and you think I'm on crack

How you answer gives away what part of the country you're from. I would've said it's a crayfish--after I screamed for someone to come kill it first--and that's what most Northerners would say. If I were writing a story down in Louisiana, there would be no freaking out, it would be called a crawfish, and you'd most likely find the thing in a boiling pot of water just before it's served up for din din.

Eew.

The point is not that this particular cuisine triggers my gag reflex, but that culture and dialogue must be reflected accurately if you want your stories to be believable. Do your research first.

And whether you're currently researching out American linguistic variations or not, here's a fun website to check out...

22 Maps That Show How Americans Speak English Totally Differently From Each Other