My dog is polydactyly; she has extra toes.
Wow, I've never heard of that before!  How many extra toes does she have?
Three.  It's unusual with dogs, though.  Cats along the East Coast of North America are often polydactyl.
Interesting!  Are there other animals with this phenomenon?
Yep.  It's a congenital physical anomaly that can also happen in humans!  
Really?  Awesome!  Are there certain breeds of dogs that have this, or can it happen with any breed?
Well, my dog is a mongrel, so maybe just mixed breeds?
Is it wise to remove the extra toes, or is it better just to leave them alone?
The extra digits are usually soft tissue so they can be removed.
Does something cause this, or does it just happen for no known reason?