A drain (first) and fill (refill) is not essential to a complete flush, but it is more efficient.

If half of the contaminating solute remains after each partial drain and diluting refill, then ½^n, when n=5 is almost 100% gone.

If you burst your guts and crap into your belly, the doctor will do this serial dilution about ten times estimating that 1% of the bugs remain after each dilution. A homeopath uses the same procedure when he gives you tincture of cyanide.

https://en.wikipedia.org/wiki/Dilution_%28equation%29

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