Galileo Thermometers demonstrate a unique method of telling the temperature discovered by Galileo Galilei (1564-1642). The Galilean thermometer works due to the principle of buoyancy, a force exerted by a fluid that opposes an object's weight. The balls inside the glass have different densities. When the temperature changes, these balls rise and fall. To read the temperature, check the group of balls near the top. The lowest ball of that group tells the temperature (within 4 degrees). If all the balls are at the bottom, it is hotter than the temperature of the top ball. If they are all at the top then it is cooler than the temperature of the bottom ball. The liquid inside the thermometer is actually a mixture of water and paraffin oil.