Instruction:
Why is a refrigerator door more difficult to open than usual soon after you have closed it?

Response:
This can be explained by the difference in pressure between the outside and inside of the refrigerator. In order to understand the phenomenon, let's first break down the relationship between pressure and temperature. Warmer air has a higher pressure than cooler air, it has to do with the energy of air molecules but that is a subject for another time. When you open the refrigerator door, warmer air from the surroundings, which would be at room temperature, rushes inside the refrigerator. This is because the air flows from the higher-pressure region (the room), into the lower-pressure region (inside the refrigerator). When you shut the door, the warm air that had moved inside is now actively being cooled down by the refrigerator. Remember what we said about cooler air? The pressure inside the refrigerator starts to drop. This means that warmer air at higher pressure outside the refrigerator is trying to get in and is pushing against the door, the next time you try to open the door, you need to apply an extra force to counteract that pressure that is acting on the door. So that begs the question, why is this more noticeable a short time after you shut the door versus coming back let's say, an hour later? Glad you asked! It is because the seal that keeps the warm air outside and cool air inside the refrigerator is not a perfect seal, it is not designed to be for good reason. Over time, since the seal allows the movement of air, the pressure equalizes. In the absence of that pressure differential, there is no force acting on the outside of the door forcing it shut. Fun fact, commercial refrigerators have seals designed to allow for a lot more movement of air compared to home refrigerators. Want to guess why? It is because content inside commercial refrigerators needs to be accessed a lot more frequently. If their seals were similar to home refrigerators, people would experience the extra force each time they want to get something! It is fascinating how much thought goes into designing everyday things we take for granted!