When Omar locked himself out, he called his neighbour. They had exchanged spare keys the previous year.
“I have yours,” the neighbour said through the phone, “but it’s in my kitchen. I’ve locked myself out too.”
A locksmith arrived and opened Omar’s door in a minute. Omar went inside, found the neighbour’s spare key, and opened the house next door.
“Now we can get your spare,” said the neighbour.
They walked into the kitchen, where his key ring hung beside the kettle.
“Our system works perfectly,” Omar said, “as long as only one of us forgets a key.”
The locksmith put away her tools. “And if both of you do, I get to meet the whole street.”