Logan figured it out.
How to go down the stairs backwards, that is. It's almost as if he watches how Emma does something for a week or so and then just decides to do it one day. Then pow! He's off to the races.
Today, he started climbing down the stairs and he did it over and over and over again, while Emma stood at the top of the stairs and went "Ehn! Ehn!".
Apparently just as suddenly she's forgotten how to do it or has determined she doesn't like him stealing her thunder.
I'm betting it's the latter.