Upon further reflection on my recent stupid injuries, I realized that the real cause was that I'm still too young, rather than too old. First of all, if I was a bit older (either mentally or physically), I probably won't be playing those silly games with the girls. Secondly, although I no longer want to win everything at any cost, I still want to win a lot of things. When I was explaining to the girls that I was hurt so bad because I fell when running really fast, Vivian asked me a question that hit the key point: "Daddy, why do you want to run really fast?" And finally, I still can run really fast. According to the formula E = (m * v^2)/2, tripping over some unknown objects can have serious consequences when one is running at a fast speed.
Anyway, my knee is good enough to carry me back to office today. As expected, everyone (except Jessie) asked me if I had a bike-related accident. I told everyone that the real story was far more embarrassing than that, and people with kids seem to be able to understand. Jessie asked me if I injured myself while having fun or just doing something silly, and my answer to that was "I was doing something silly to have fun at that time".