Oo-er.
That is bizare.
Mind you, I came up with our own half weight water idea.
I bought a 25ltr container then filled it half full as then 12.5ltrs weighs half of what 25ltrs would.
When it was empty, I half filled it again so it still weighed half of what 25ltrs of water would.
At the end of the day, we'd used 25ltrs of water that never weighed more than 12.5ltrs of water.
Genius or what?