Trapped air pocket.
cold water sinks to the bottom, and hot water rises to the top
the top part is cold because that's where the freezer is and the refrigerator is the bottom part.
The refrigerator is cold at the bottom and warm at the top because warm air rises. Cold air is heavier than warm air and settles toward the bottom of the appliance.
NO
The thermostat has not opened yet.
radiators are cold on the bottom and hot on the top?
I assume that you are referring to hot water radiators, heated by an oil furnace. These radiators could have air in them and will not allow the hot water to fill them and give off any heat. If these are connected to the same hot water line that feeds the other working radiators, the in and out lines and the bottom on the cold ones should be hot. These radiators will then have to have the air bled out of them. They should have a valve on the top somewhere to do this. If there is no heat in the lines there must be separate line to that room that is plugged or shut off.
I think not because cold water is heaver than hot water and hot water rises to the top and cold drops to the bottom.
Warm gases and warm fluids rise to the top. Same principle that makes hot air balloons fly.
Cold will sink to the bottom. Hot water will rise to the top.
The cold water supply enters the tank at the bottom near the burner. Heat rises, as does hot water. The hot water supply is piped out from the top.
On the dipstick there is a safe range. You can check it either hot or cold. When cold the level should be close to the bottom of the safe range. When hot it should be close to the top of the safe range.
You will get warm instead of hot water. There is a tube inside the tank on the cold side that goes to the bottom of the tank and the hot comes directly from the top. Reverse them and you are going to get the cold water that settles to the bottom of the tank. It will not hurt the tank or anything, it just makes for poor hot water.
cold water sinks to the bottom, and hot water rises to the top
Air may get trapped in a radiator, leaving it cold at the top whilst it is warm at the bottom. It then needs 'bleeding'. Radiators may also have the opposite problem, where they are warm at the top yet cold at the bottom. This is caused by a build up of sludge in the radiator and will probably require professional help. Sometimes the downstairs radiators stop heating up, whilst the upstairs ones are fine. This is caused by a problem with the pump and will again require professional help.
Because - hot air is lighter than cold air - and thus it rises.
the top part is cold because that's where the freezer is and the refrigerator is the bottom part.