The challenge...and the response

Someone who truly understands dog behavior, like Patricia McConnell whose advice you can read on her blog here,  might disagree with me, but this looks like a challenge to me.

See how Darby is staring straight on at Bonnie:

Here, she is picking up her right front foot, about to take a step forward:

Step taken.  (Notice Bonnie's tail--straight up and still.)

And, while I wasn't quick enough to snap a photo at the exact moment Bonnie responded, this is what followed:

It must be fun; they do it all day long.  Inside and out.