FLOSS EXPIRATION

I went to see the dentist yesterday for my routine semi-annual cleaning. All went well and I headed to work afterward. I always feel like a little kid when they give me the treat bag as a parting gift. I always take the floss to work and toss it in the top drawer of my desk. 
I am a floss-oholic, I floss several times a day. But this afternoon as I was putting my floss away, I noticed that there is an expropriation date on the floss. Why? I wonder if it will stop working if I keep it that long. Will it just refuse to do its job? I wonder!