One of my jobs at work is to determine the amount of oil and grease present in wastewater samples. In the beginning of the procedure I usually take a tray of oil and grease cups out of a 125 degree oven and place it in a desiccator for roughly an hour before weighing on a balance. After extracting the oil and grease out of the water and into the cup it is again dried in an oven and placed in a desiccator to cool before weighing back. The first cup in the tray I use as a constant which has to be within + or - 0.0005g of it's initial weight. If I leave the cups cooling too long, my first cup will be up to 2-3mg heavier than where it started. If I pull them too early then my constant is light. I realize this is a very basic question but why does this happen? Is it because the heat transferring to the gas surrounding the cup makes the gas rise and therefore exerts less pressure down on the cup when it is hot? When the cup is cold it can't excite the gas away as well?