Rachel and Quinn sing a medley of tracks from 'West Side Story' and TLC on 'I Feel Pretty / Unpretty,' a song from tomorrow's 'Born This Way' episode of 'Glee.'

Accompanied by acoustic guitar, Lea Michele and Dianna Agron, as Rachel and Quinn, harmonize elegantly on 'Unpretty,' the hit TLC ballad from 1999 that addresses the insecurities that can result from societal expectations of female beauty.

As the girls sing the line about feeling so damn unpretty, they interject a portion of the 'West Wide Story' number 'I Feel Pretty,' singing, "I feel pretty, oh so pretty / I feel pretty and witty and bright."

The song builds steadily until strings appear and the choruses of both songs overlap. The final line is "I feel pretty... But unpretty," reflecting the conflicting emotions many people have about their physical appearance.

Combining 'I Feel Pretty' and 'Unpretty' is a clever idea, and the execution is impressive. The duet appears to fit in perfectly with the episode's theme of self-acceptance, which will also be reflected in covers of Lady Gaga's 'Born This Way' and Sammy Davis, Jr.'s 'I've Gotta Be Me.'

Rating: 9/10

Listen to Glee Cast (Rachel & Quinn), 'I Feel Pretty / Unpretty'