Can anyone tell me what DBSK/TVXQ song is playing the background of this video?

http://www.youtube.com/watch?v=5g6AEqxbMr4
Yeah, Xiah is cute and all... but I'm more concentrated on the
background music right now. I used to love this song when I was really
young and it seems so familiar but I completely forgot the title...
can anyone help me? :D