See the water demonstration of the Pythagorean theorem:

[[![Water demonstration of the Pythagorean theorem][1]][1]][2]
https://www.youtube.com/watch?v=CAkMUdeB06o


  [1]: https://i.sstatic.net/TGAih.png
  [2]: https://www.youtube.com/watch?v=CAkMUdeB06o