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.