21st birthday is a very special occasion to celebrate. After having spend two decades on this Earth, make the stepping stone to the third decade on earth for your loved ones special by coming up with some very special and unique 21st birthday presents for them. A unique gift idea for a 21st birthday would be Galileo thermometer. A Galileo thermometer comprises of a sealed vertical glass tube with a clear liquid which is more often than not water. This water is inside the glass tube. Inside the glass tube can also be found a lot of brightly coloured glass bubbles which are calibrated and keep floating at different levels inside the tube. While some bubbles would be found floating at the top of the tube, some others are found sitting on the bottom of the tube. The bright colours of the bubles primarily serve the purpose of decoration, and are not indicative of the temperature at all. The glass bubbles consist of a liquid which is usually a combination of colouring, alcohol, and water.
With the change in the room temperature, the temperature of the water which is there in the tube also keeps changing. This change in temperature makes the water to contract or expand. As the volume with regards to water keeps changing, its density also changes. This thermometer works on the basis of Archimedes