Roo crosses the bridge, just because.

Generally, and especially not on hot days, Roo does not use bridges. She doesn't believe they're anything but a cruel device created by humans to keep dogs out of the water, where they belong. Under this bridge, a fresh stream was running nice and cool. Roo, who knew it well and had been in it minutes before, was therefore crossing this bridge only for the challenge. I tried to talk her out of it, but she just had to find out for herself whether she could do it. 

Any time she achieves a risky goal, like jumping off a cliff or investigating something that looks scary, she laughs and trots a few steps. But in this case, she was clearly embarrassed. She slinked away and never tried it again.