Do you own one of these water bottles?
I love mine and use it all the time. I am very careful to hand wash it and make sure it doesn't make it's way into the dishwasher because I have heard it's bad for the plastic. I always take off the straw and make sure it is clean but I never thought about taking off the blue mouth piece... until today.
I was playing with it and realized that when I pulled off the mouth piece that it was full of gross black stuff. I am not 100% sure what it is but I know it is bad news.