...you're working at your computer and you have your fingers ready over W, S, A, D(or preferred controls) and fingers on the mousebuttons ready to dodge a surprise attack (Especially if that Clippy thing sneaks up on you).
...you start pretending you're in Quake 3 or Unreal Tournament while taking a tour of an ancient castle.
...you wonder how high you could rocket-jump in real life.
- These are examples from my everyday life. Do you have any?