stunt riders?

Somebody taught himself to ride a bike this week.

And then big sister, who has approached her bike with serious trepidation since the training wheels were removed, hopped on like it was no big deal and they rode circles together for four hours.

Also, I overheard this:

C: We're really good at riding bikes.
E:  Yeah
C:  And we're getting better every day.
C:  We're stump riders.