Burning PoA question

[Herald Talia]

No: HPFGUIDX 25204

If this has been answered before, please punish me. I've been (slowly)
reading through old posts. 
At the end of PoA, Sirius sends Harry a permission slip to go to
Hogsmeade. How does he know Harry needs one? They really didn't have much
time to talk at the end of that book. 
Robyn

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Donald heard a mermaid sing, Suzy spied an elf.
But all the magic I have known,
I've had to make myself- Shel Silverstein
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