The longer a conversation lasts, the higher the chance of Team Fortress 2 being mentioned. The strange thing about this law is that no matter how long the conversation actually lasts, there is a 100% chance that TF2 will come up.
Me - "Hey Thomas. What did you think of that Walking Dead episode last night?"
Them - "Why would I have watched it? You know that I played 69 hours of TF2 last night!"
Me - "Really?! You're providing a textbook example of Thomas' Law!"
Me - "What's wrong with you? You're acting very aggressive today."
Them - "Aggressive? I think you mean Heavy-like!"