For those of you who don't know, Celtic and Liverpool's fans are known world wide for their passion and love of the game. The thing both have in common is "You'll Never Walk Alone". You'll Never Walk Alone (or YNWA for short) is the song that both Celtic and Liverpool fans sing before all their home games. The song was written for the play "Carousel" but it wasn't adopted by Celtic and Liverpool fans until British group "Gerry and the Pacemakers" sang a version of it in 1963.
There's a friendly arguement between Liverpool and Celtic fans over who started singing the song first, and no one really knows for sure.
YNWA is a great traditioin with both teams and listening to the fans sing it is really amazing.
Here's a video of Celtic fans singing it before their game with Barcelona in 2004.
Here's another video of Liverpool and Celtic fans singing together before their Uefa Cup semifinal match in 2003. Celtic won the match 2-0.
They're really awesome videos, and they give you a sense of how much pride the fans take in their teams and how passionate they are for the game.