* Thermometers use thermal expansion: The liquid inside a thermometer (usually mercury or alcohol) expands when it gets warmer and contracts when it cools down. This change in volume is what allows the thermometer to measure temperature.
* Gravity keeps the liquid in place: When the thermometer is upright, gravity pulls the liquid down, keeping it at the bottom of the bulb.
* Surface tension helps too: The liquid in a thermometer also has surface tension, which helps keep it from spilling out when turned upside down.
So, why doesn't the liquid move when turned upside down?
The reason the liquid doesn't move is because of the capillary action within the thermometer. The narrow tube of the thermometer has a slightly adhesive surface, which pulls the liquid up. This is why the liquid is able to climb the tube, even against gravity.
While the liquid might appear to shift slightly when you invert the thermometer, it's just a matter of the liquid redistributing itself within the tube. It doesn't actually flow down the entire length.
Important Note: Modern thermometers are often filled with alcohol or other colored liquids instead of mercury because of the toxicity of mercury.
