random silly question

[suziannabean.livejournal.com]

I am just getting into altering a book - and I know that acid is bad in paper (or at least the acid-free paper is always so excited to be acid-free ...), so that is what I am definitely looking out for ... but what does the acid do? Why is it so bad?

(I'm sorry if this is redundant - I did look in the memories, but it is such a random question that I didn't find it there!)