Kiss (aka A Prelude to a Kiss) - S1-E3
Continuity mistake: After Joey finds the visitor Anderson playing a violin on his yacht, they have a back and forth conversation where she hides her work apron behind her back. When the camera angle switches back and forth, her arms are in different positions. (00:14:10)
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.