There are 2 supply lines that connect under the sick to the faucet, one with cold water and hot water that’s been heated in a hot water heater. The ratio of hot to cold it determined by the faucet, adjusting the temperature.
The toilet only uses cold water, so a flush can divert water from the pipes to the toilet and momentarily reduce the cold water going to the sink — throwing off the ratio until the pipe’s pressure adjusts to supplying 2 fixtures.
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