I was recently asked this following interesting question:
How can you find what is the direction of the water flow inside a horizontal pipe, given the pipe is not transparent?
- You can assume the pipe is metallic, but I would also love to hear solutions that don't assume that (if there exist any).
- You cannot see the start or the end of the pipe, and you can't pour anything inside.
- I am familiar with the heating solution, so I would appreciate any other solution.
- Is there a way to do this with magnets? If so, how? This essentially was my first thought, but I didn't actually think it through because I'm not really sure what is going to happen there.