So this sent me down the wikipedia rabbit hole :D One thing I never realised is how dramatically the viscosity of water changes with temperature. This is probably why you can recognise hot water from a cold water in a video. Temperature (C) / Viscosity (mPa*s) 0.01 1.7911 10 1.3059 20 1.0016 25 0.89002 30 0.79722 40 0.65273 50 0.54652 60 0.46603 70 0.40355 80 0.35405 90 0.31417 99.606 0.28275
I've sometimes thought that the sound of water in my shower changes as the water reaches max temperature. Perhaps that's why?
I was always able to tell as a kid in my childhood home, likely plumbing-system dependent.