Have you ever seen a Galileo Thermometer? They are a pretty cool way of telling what the temperature is and it also serves as a cool decoration for your home. The thermometer has little glass bubbles with different color liquid inside of them. Each little bubble has a tag on them with a different reading of the temperature. You read a Galileo thermometer by reading the tag on the lowest bubble that is still floating. The way the thermometer works to change to different temperatures involves a bit of physics.
An object immersed in fluid experiences two forces, the downward force of gravity and the upward force of buoyancy. In the Galileo thermometer, its the downward force of gravity that makes it work. Each of the tags on the different bubbles has a different calibrated weight, making each one a slightly different weight from the others. The liquid inside the each of the bubbles has the same density, so that when the weighted tags are added, each bubble has a slightly different density then the others due to the ratio of mass to volume. The density of all of the bubbles is very close to the density of the surrounding water. Therefore, as the temperature outside the thermometer changes, the temperature of the water the bubbles are immersed in also changes. When the temperature of the water changes, it either expands or contracts which changes its density. So at any given density, some of the bubbles will float and others will sink. So for example, if the temperature is increasing, the waters density decreases. So the bubble with a tag that says 72 degrees, for example, will have now have a weight per unit volume that is greater than that of the surrounding water rather than lighter, and it will sink to the bottom. Very neat!