I Could Have Danced All Night from My Fair Lady is featured in Mash-Up, the eighth episode of Season One. It is sung by Emma in a wedding dress shop while shopping for a wedding dress that she can dance in. It is her song choice for the first dance with Ken at their wedding.
This is Emma's first solo. It's a cover of the song from the musical My Fair Lady and was released as an exclusive track on Glee: The Music, Volume 1. During this musical number, Emma's feelings for Will are evidently clear, and are reinforced when the music stops and she is left standing, in what seems like shock at her feelings, and the closeness between the pair. This is the first time we hear Emma sing a full solo.
Emma:
Bed, bed
I couldn't go to bed
My head's too light to try to set it down
Sleep, sleep
I couldn't sleep tonight
Not for all the jewels in the crown
I could have danced all night
I could have danced all night
And still have begged for more
I could have spread my wings
And done a thousand things
I've never done before
I'll never know what made it so exciting
Why all at once my heart took flight
I only know when he began to dance with me
I could have danced danced danced
All night...