Stacking cups or glassware in cabinets may save storage space, but it doesn't always save time. Trapped moisture and air within stacked cups often causes them to stick together. Attempting to separate two stacked cups with just bare hands is sometimes time consuming and occasionally impossible. Applying extreme force to two drinking glasses while prying them apart can also cause them to break and cut your skin. Fortunately, a basic technique can separate two cups stuck together.
Fill the top cup halfway with ice cubes. Sit the bottom cup in a bowl of hot water. Submerge half of the bottom cup in the hot water.
Let the bottom cup sit in the hot water for 5 minutes. Chill from the ice cubes causes the top cup to contract while heat from the hot water makes the bottom cup expand.
Remove the bottom cup from the hot water. Pour the ice cubes out of the top cup.
Attempt to pry the two cups apart with your hands. If the two cups remain stuck together, repeat entire process.
Trickle cooking oil in between the two stuck cups to help loosen them.
Do not use boiling water; you could scald yourself.