"you didn't offer a fee" is misleading; the bitcoin software 99+% of people are using today doesn't let "you" very much flexibility in how you offer fees. So basically, 99+% of people using bitcoin are charged fees based on a set of built-in rules.
If you want to send a fee with every transaction, you can run bitcoin with the -paytxfee= option; there isn't much reason to do that now, unless you're generating lots of very small (few-bitpenny) transactions. The Bitcoin Faucet is running with -paytxfee=0.01 because it DOES generate lots of very small transactions.
From the wiki:
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