Challenge: A Good Chemistry Discussion Question?

Here, Ellen has laid down the challenge of coming up with a good chemistry question–one that will stimulate interesting discussions like the rainbow question . So I'm giving that challenge to all of us.

If you're reading this, consider doing one of the following in the comments:

• Let us know a good chemistry question you or a colleague has used
• Brainstorm one or two possible questions (just throw 'em out there, see if they stick)
  
• Toss out some topic areas that you think might be a rich possibility, and say why
  
• Contribute a question about chemistry that has intrigued you personally
  
• Suggest a demonstration, situation, or phenomena that might induce a good question
• Suggest some essential features of a good chemistry question
• React to someone else's contribution
• Invite a colleague or friend to contribute
  

To help maybe start us off, here is a chemistry-related question over at dy/dan that led me and several of my colleagues to dig in for a few hours. My office white board is full of drawings, equations, arithmetic, etc. If you wanna know how that question might go in a high school classroom, there's a gallery of student whiteboards about the question over at Newton's Minions.