How does a thermometer work?
A thermometer turns a temperature-dependent physical change into a readable value.
Liquid thermometers
A liquid thermometer uses thermal expansion. As temperature rises, the liquid expands and climbs through a narrow capillary, turning a small volume change into a visible displacement.
Why mercury became less common
Mercury gives a clear column and predictable expansion, but toxicity concerns led to its removal from many consumer applications. Coloured organic liquids are now common.
Digital thermometers
Electronic thermometers use sensors whose electrical properties change with temperature. A thermistor, for example, changes resistance; electronics convert that signal into degrees after calibration.
Measurements take time
The sensor must exchange heat with what it measures. Until the temperatures come close enough, the reading keeps changing, so response time depends on sensor size, design and thermal contact.
What to notice in real use
Real devices are never ideal. Response time, insulation, sensor placement, seals, manufacturing tolerances and calibration all affect what you observe. Two appliances based on the same physical principle can therefore behave differently.
That is useful rather than confusing: once the core mechanism is understood, the remaining differences often come from engineering choices rather than from a different law of physics.
Look at the mechanism, not just the object
A useful way to understand an everyday object is to follow a chain: what enters the system, which material or component responds, what physical or chemical change occurs, and what useful effect comes out. This separates the scientific principle from the casing and design.
The same principle often appears in very different objects. Once the mechanism is recognised, links between chemistry, materials, electricity, heat and mechanics become much easier to see.