When football players sign on the dotted line of a professional contract, they are expected to give their body and soul to fight for the club they represent, fight for the name on the front of the shirt.
But at times it can be difficult to play with their heart because they grew up supporting another club, possibly even a major rival. But as professionals, they are asked to put that aside and focus on the task at hand.
It is difficult; imagine trying to support another club, even a derby rival. These players have lived through it all.
#1 John Terry: Manchester United
Captain. Leader. Legend. That's what his moniker is at Chelsea. And why not? Terry is a player who spent nearly two decades in the Blues senior team and captained the club to five Premier League titles and a historic Champions League title.
So you'd expect him to bleed blue, right? Well the fact of the matter is that he supported arch rivals Manchester United when growing up.
One can't blame him. It ran in the family. His grandfather and father supported the Red Devils and the love for the Old Trafford club was passed down to him.
"I was Man United growing up. You know what it's like when you're a boy, you want to support the team that's winning everything. And my dad and granddad were Man United fans.
"But my first day at Chelsea I knew that was the club I wanted to be at. I just loved it. But Man United also approached me.
"I met Alex Ferguson - great to meet. During the school holidays I would go up to Manchester and train with their boys for two weeks, play a few games and come home."
Terry had even uploaded an old photograph where he was sat beside Sir Alex Ferguson and his parents.
The former England international was only 14 at the time and played as a holding midfielder. And although he loved the club, he decided that Chelsea was the right way to go.
"I think it was hard for my dad to take that I was going to sign for Chelsea," Terry said. "But I had to sit him down and tell him it was definitely the right decision."