How Sylvie, whose feet insist on jerking down to the safety of the sidewalk whenever she tries to ride her bike, finally gets angry enough to peddle furiously. . . and so finds herself on a downhill course ""speeding along surely, swiftly, with no fear at all."" Earlier, Virginia has zipped by taunting ""Can't you ride?""--and now Sylvie tosses the same question to little Georgie, who is still trying to learn. We'd prefer more magnanimity in triumph, but Anderson's pictures give disgruntled Sylvie's endeavors a catchy, expressive, individual slant.