[jla]

Cold does not exist. It cannot be measured or observed. What people call cold is actually a lack of heat

What are those tubes filled with mercury? One cannot observe water changing to ice?

[ShadowAce]

What are those tubes filled with mercury?

Those are thermometers. Tools to measure heat. Hence the prefix thermo.

One cannot observe water changing to ice?

One can observe water changing into ice--the movement of heat away from the water causes that.