|MadSci Network: Physics|
Ruth thanks for your interesting question. The answer is fairly straight forward. When milk is taken out of the fridge (icebox) it is cold. When it is left to stand in a warm kitchen the air inside the milk container will warm up and expand. When this expanding gas gets enough pressure it will be able to just lift the lid. Some of the air escapes lowering the pressure in the container. The pressure is now too low to lift the lid. After a short time the pressure builds up again, and so on until the temperature of the container is the same as your kitchen. This process is unlikely to happen with full containers of milk. You have to have air present. You can compare this to a tyre on an automobile. It expands when warm and contracts when cold. This happens to a very small extent with solids and liquids too. A thermometer is an example of liquid expansion. An oven shelf is deliberately made small to allow for expansion in a hot oven. I hope this makes sense. All the best Sean Hunt (Science Teacher)
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