Musically, "Eight" has been described as a "nostalgic" pop rock song with lyrics that find IU confessing as a twenty-eight year old, using a virtual figure and various analogies.

The accompanying music video for "Eight" was directed by Kim Woogie (GDW) and features IU revisiting various phases of her youth through her memories.

On April 27, 2020, EDAM Entertainment announced that IU would release a new digital single on May 6, 2020, which would be a collaboration with Suga from BTS.

[8] The photo shows the singer in a faded Medieval dress, staring outside the window anxiously with blank eyes.

[10] It shows a round reel tape playing constantly and a snippet of the song's lyrics "So are you happy now" was heard towards the end of the teaser.

[12][13] Set in a "futuristic" backdrop, it shows IU dressed in white, entering a laboratory and then laying down on an examination table.

"[20] While I'm not sure whether it’s from my personal emotion or from the overall social atmosphere coming from the disaster, my 28 would be remembered by the recurring feeling of lethargy and longing for the 'orange island' where we felt freedom rather than sadness.

[28] It derives from piano and guitar instrumentation and incorporates Suga's characteristic "airy" synths in its production for which the song invokes "feel-good" and "nostalgic" vibes.

"[20] Teen Vogue's Sara Delgado described "Eight" as "a narration between two characters in soliloquy form" that "dwells on memories and past encounters" and felt that the various analogies in the song are used "to transmit a sense of longing, all through the lens of youth and loss.

[26] "Eight" debuted at number one on the week 19 issue of South Korea's Gaon Digital Chart for the period dated May 3–9, 2020.

[39] Six months after its release, "Eight" was certified Platinum by the Korea Music Content Association (KMCA) on November 12, 2020, for surpassing 100 million domestic streams.