When is a species not a species? Pretty much always, because it turns out, biologists have no clue what a species actually is. We think we know most of the time, but nature keeps throwing us curve-balls. There are so many exceptions to any rule you come up with to define a species, that it becomes impossible to make a rule. You might have thought this was pretty basic stuff, that we figured it out a long time ago, but... you'd be wrong. Learn what a complex topic this really is and why it has had biologists scratching their heads since day one on this episode of The Cliff's Edge.