This is a vapor pressure experiment, the jar was sealed while the water was boiling on a hot plate, so the steam displaced all the air out of the jar. After sealing, the jar was quickly removed from the heat and let cool. now, there is only water and water vapor in the jar (give or take, there is some hair and dust in the water, and probably some remaining dissolved gases). The pressure inside is always equal to the vapor pressure of the water, which depends on temperature. At room temperature, it's almost a vacuum inside, and as the temperature increases, so does the pressure. At 100C, the pressure inside is one atmosphere, which is the same condition under which the jar was sealed. Whenever there is a heat transfer from the bulk water to the lid (condensing the vapor), the water boils, no matter the absolute temperature. In this case, the water was luke warm, and I could hold the jar while it boiled inside. Another interesting property is how easily the water cavitates. simply moving the jar around or tapping lightly on the top causes the water to separate from the walls of the jar, and it makes a clapping sound.