What is the most interesting chemical reaction you have ever witnessed?

Question: What is the most interesting chemical reaction you have ever witnessed?
- Keywords:
  experiment,
  reaction
Asked by anon-248477 to Connor on 17 Mar 2020.
- Connor Prior answered on 17 Mar 2020:
  
  My colleague did a Vilsmeier-Haack reaction (very famous reaction and it changed colour about 4/5 times in one go from green, to yellow, to red, to blue, then finally orange!
  
  https://www.organic-chemistry.org/namedreactions/vilsmeier-reaction.shtm
  
  Here is a link to what the vilsmeier-haack reaction is! You’ll learn about this at university!

Question: What is the most interesting chemical reaction you have ever witnessed?