The Galileo thermometer consists of a sealed glass tube that is filled with liquid and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. Attached to each bubble is a little metal tag that indicates a temperature. These metal tags are calibrated counterweights. The bubbles are calibrated by adding a certain amount of fluid to them so that they have the exact same density. So, after the weighted tags are attached to the bubbles, each differs very slightly in density (the ratio of mass to volume) As the temperature of the air outside the thermometer changes, so does the temperature of the liquid surrounding the bubbles. As the temperature of the liquid changes, it either expands or contracts, thereby changing its density. A narrow spout connects to the body below the liquid level and rises above the liquid level, where it is open to the atmosphere.