I was reading an article about rehydration which said to weigh yourself before and after a run to work out how much water you have lost during the run.
So - this morning in hot humid conditions I did 40 minutes of moderately hard 5x1klm repeats ( with a minutes rest between each) and 10 minutes of easy jog/walk to/from the running track. The moderately hard really turned into hard because of the conditions which caused me much sweating.
Anyway , when I weighed myself upon return home , I was 1 Kg ( 2.2 lbs) lighter!!! Of course , if it had not been so hot/humid and if my run had been a slow easy long distance run , that figure would be different. But it is a handy reference to remember what rehydration is needed after such runs ( and during long runs)