Six fingers! It happens sometimes — especially to a few choice species, including humans. It's called polydactyly. And there's an amazing story behind how we tracked down the reason people, cats, and flies seem to grow extra digits.
The 1960's were a much simpler time — a time when cowering beneath your school desk was still considered a good way to protect yourself in the event of a nuclear air strike, and a child palming a decidedly phallic object could just as easily pass for having a sixth finger. Wait, what?