He only opens a window
The puzzle
Every morning a man opens his window.
That is all he does — and every morning the neighbours check that he has.
See the answer (spoiler)
Spoilers from here on. If you want to work it out yourself, leave this folded and move on.
Answer
He is elderly and lives alone.
The open window is the agreed signal that he is all right today.
If it is still shut by mid-morning, somebody goes round and knocks.
