This has always annoyed me because I don't understand it.

Let's say you open a pack of biscuits, but before you put it back into
the cupboard you wrap it in "cling film" to keep them fresh (I think you
Americans call it "saran wrap").

Now when you wrap something in cling film, you're doing either one of
two things:

1) Making it air-tight
2) Not making it air-tight (you're restricting the air flow into it)

If something were to be wrapped air-tight, then I could see how the air
within it would eventually become exhausted (I think bacteria take
oxygen from it or something like that), and so the decay process would
become retarded as soon all the oxygen's depleted. I could understand this.

However, when you normally wrap a packet of biscuits, you're not nearly
making it air-tight. I'm very surprised that wrapping it even has an
observable effect on the air flow into it (considering it's sitting in a
cupboard and there's no draft), so I'm even more surprised that it
actually has an effect on whether the biscuits will be soft when you go
to take them from the cupboard the next day.

Is there something I'm missing here? Does the cling film really have
such a huge effect on the airflow into the biscuits?

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