I have quite a few of these but I'll just start with one. Back when first graders were still learning about matter, we were having a discussion reviewing what we knew about liquids. I asked for examples and I got: water, juice, milk, soda, etc. I saw that they were only naming liquids that we could drink. So I asked the students if they could name a liquid that we weren't supposed to drink. And the first student to raise his hand said very matter-of-factly, "Beer!" Can you share any stories of your own?