After they finish doing the fillings, they file the new material down to fit your bite. You might recall them having you bite down on a piece of paper and then they'd sand away all the parts that were hitting. The more fillings that get done at once, the harder it is to get this done properly, I think. If certain parts of your filling or teeth are hitting before the other parts are when you bite down, or if they are otherwise not fitted well, it can cause pain throughout. (They are also more likely to fall out then)
Source: Personal experience with bad fillings.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.