I discovered my son drinking out of this late Monday:
It's a kids' Tupperware pitcher. At first, I was thinking it was a sign that the dishes had to be done--I'd had 9 kids here Monday afternoon and they all used a plastic cup for a drink of water. So, when my son went to get a drink of water, there weren't any plastic cups left, so he grabbed this.
It hit me later that we did have glass glasses in another cupboard. And mugs. We had proper things to drink out of.
And so, the burning question in me: Is his use of the pitcher a creative solution to a problem or just plain laziness? ;)