ETA: Okay, I’ve recently found out that this quote is not Vonnegut, although he did write something similar to this in a letter to schoolchildren. That letter was posted to three–ring’s blog with her commentary posted below. Her commentary was copy-pasted to Facebook as being written by Vonnegut, and it’s been passed around that way ever since. Proof here.
I’m fixing this post with proper attribution. I did do a quick web search before posting this, but apparently it was too quick. Bleagh.

You lost over 100 lbs!? As long as you’re not unhealthily underweight, that’s absolutely incredible!
Thanks, still got about 30-50 to go. It’s getting tougher.