its where you gurantee that it will rain again

No, seriously, I've heard of this and its a nifty idea.
I think its where the window cleaner in question offers to come and clean the windows again free of charge if it rains within 24 hrs of him doing them.
Good idea because from the custies point of view its a safe bet, but how many will actually take it up? Hardly any, even if they have paid more for it.