“백열등 (Light Bulb)” is a rap-heavy, love-centered piano ballad that appears on megaproject NCT 2020 : RESONANCE Pt. 1.

The track features Taeyong, Doyoung, Kun and newcomer Sungchan in the rotational unit NCT U.

The song compares letting go of love to a light bulb—one that, when switched off, just comes back on from thoughts of happier times in the past.