really? someone asked this? Short answer, through the pipe's that run through your house. That would be inside walls or under floors. before that point it comes from a underground main line that runs...
Plain Tap water would freeze fastest.
Adding salt or sugar to tap water will cause a depression/decrease in freezing point. Hence it will be harder to freeze the tap salt or sugar water.
It is probably due to a blockage somewhere in the coldwater pipe or faucet. The easiest to check is the faucet. Turn off the water supply valves. Then take apart the faucet and look for tiny pieces...