Why does 'fortune' write on the fortunes.dat file?

[Felix Lee]

"fortune" doesn't pick a random fortune to print.  Strange but true!
What it does is print the next fortune that fits your command-line
options.  The next fortune is determined by reading (and writing) a
counter in the fortunes.dat header.  The randomness is in the order of
the string table.
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Felix Lee	flee at shire.cs.psu.edu	*!psuvax1!shire!flee