# Is my electric kettle collecting old water during the week?

I have an electric kettle at work. On Monday, it's empty and I pour in about 1.5 liters of water. I usually end up drinking about 1 liter per day and refilling 1 liter each morning.

At the end of the week, I rinse it out because in my mind, there is .5 liters of "old" water from the beginning of the week on the bottom. Is that true? Or is the water constantly mixed around from being used and then refilled?

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After the $n^{\mathrm{th}}$ day, you have $1.5/3^n$ L of Monday's water left, $1.0/3^{n-1}$ L of Tuesday's water left, and $1.0/3^{n-k}$ L of the $k^{\mathrm{th}}$ day's water left. For the particulars you gave, that leaves you with 6.17 mL of Monday's water 12.3 mL of Tuesday's water, 37.0 mL of Wednesday's water, 111 mL of Thursday's water, and 333 mL of Friday's water, left over after you have poured Friday's tea.