If you want a obvious answer, tap water. Tap water is freshwater usually mixed with a tiny bit of fluoride.
Faucet.
I assume you mean when you turn on the sink water comes up from the drain in the tub? If that's the case, the drain from the tub is clogged. The sink is higher than the tub so the water comes up there.
Call a plumber
The reason the pennies sink in water is because of an idea called density. The pennies have more density than the water, and so the pennies sink. Anything with more density than water will sink in water, but other objects that have less density than water will float.
Gray Water
Energy comes in many different forms, but not usually in a form that you can say that it sinks or floats.
There is no 'is' between properties and sink, but I'd say, semi detatched, detached, manors etc, but not terraced properties
A blocked pipe. The water can not go down the pipe, so it comes out in your sink.
There are many things that one can do with an under sink water heater. The most plausible thing one can do is use it to heat their water so that hot water comes out of the faucet when the correct knob is turned.
Sink water is typically sourced from a city's public water supply system or a well. It goes through various filtration and treatment processes to ensure it is safe for consumption before reaching your sink. When you turn on the faucet, the water flows through the pipes in your home and comes out ready for use.
the drain line is completely or partially blocked, the drain line needs to be snaked out. the disposal is forcing what water it can down into the pipes but if there is an obstruction the water has no place to go but back into the sink.
Maple syrup will sink but water is unable to sink because it is water.