I've searched the google archive for an answer to this question before
posting but unfortunately to no avail. So ...

I've just secured a bookcase to the wall with two screws. The screw on the
left just keeps on turning when flush to the wood whereas the screw on the
right has become tight. I've often wondered:

1. why does a screw sometimes keep on turning?
2. if a screw does keep on turning, what is the impact on the security of
the fixing?

Would be grateful for any help.

Thanks.

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