100 Light Years From Home - S5-E19
Audio problem: When the band M to M is performing at the MTV concert, the girls start singing a song that requires a very tight harmony. During the entire time they are singing together, the camera shows close-ups of both girls. But at one point, they show the second girl with the guitar simply smiling and looking up. She is *not* singing at all, yet the song continues to have both their voices in the harmony. The rest of the bandmembers are male, and the harmony was formed by the two female lead singers. (00:13:30)
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.