Downtown Crossing (2) - S5-E15
Plot hole: When Joey suddenly regains consciousness in the ER, she is sitting upright on the end of a hospital bed or gurney. But this doesn't make logical sense, because if she were really unconscious she would be laying down. Yet it's made very clear in that shot, that she is opening her eyes for the first time after fainting and being unconscious.
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.