I went online and found out a lot of information about boiling water. If you cover the tin and allow the water and air to cool down, the air will cool and contract, lowering the pressure in the tin. If it lowers enough, the partial vacuum will collapse the walls of the can! The boiling point definitely reduces with air pressure. There is a famous demonstration where boiling water or coffee is drunk, the trick is it was in a vacuum chamber where it boiled at room temperature. The actual definition of the boiling point of water is the temperature when the vapor pressure of the water equals the atmospheric pressure (the pressure of the air pushing down on the surface of the water). In the liquid, the water molecules are attracted to one another by their very nature and further held together by the pressure of the atmosphere pushing down on the surface of the liquid. As they are heated they gain energy to begin to pull away from one another. When they gain enough energy, they are able to separate and to overcome the force of the atmospheric pressure and form the gas (water vapor) that makes up the bubbles you see in the boiling process. The standard temperature at which this occurs is 100 C or 212 F. In the very early stages of heating, you may see some small bubbles even though the water is only warm. This is some air that is dissolved in the cold water and is being expelled by the heating. This is not boiling. As the water nears the boiling point, some areas of the container close to the heat source may be hot enough to cause some boiling in that area which produces some local bubbles. To be sure that all of the water is "at the boiling point," you should wait until there is a large amount of bubbles rising to the surface. The bubbles in boiling water are made of, air and/or water vapor, depending on how much air has been dissolved in the water.