The base of a Galileo thermometer is filled with regular water. The colored orbs inside of the thermometer are glass bubbles filled with colored water.
A Galileo thermometer is an accurate thermometer. Today, however, these thermometers are mostly used for decoration. Forces of gravity and buoyancy act on objects submerged in water. As the temperature of the water rises or falls, the objects inside becomes more or less buoyant. The glass orbs have weights attached to them with the temperature number engraved on them. The highest temperature orb on the bottom of the thermometer is the closest to the actual temperature.