I suspect Reedwalker has it.
If a tank type water heater is hooked up backwards, the cold water dip tube that drops down to the bottom of the tank will suck the coldest water from the bottom, while the hottest water at the top of the tank stays there. More volume would tend to push some of that hot water from the top down to the bottom where it will get pulled into the dip tube, and voila! hot water.
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