You have two pieces of string. They are of different lengths and different materials. All you know is that each one, when lit on the end, takes exactly one hour to burn up completely. You can't assume that either burns at a consistent rate; for instance, one may burn quickly at first and slow down near the end, and the other may (or may not) do the opposite.
Using only the strings and some matches, how do you measure 45 minutes?
[Hat tip to Darren.]
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