Surprise fire drill
I have to schedule my company's first ever fire drill. It must be a surprise, but unfortunately everyone in the office already knows that it will take place at midday, on one day in the first week of July. So I've been told to pick a day in the week that no-one will expect, so that it is still a surprise.
So, it cannot be held on Friday. Because if it gets to Thursday and it still hasn't happened, then everyone will know to expect it the following day. And then when it happens, no-one will be surprised. So Friday is out.
But then of course, once I've ruled out Friday, I'll have to rule out Thursday. Because if it hasn't happened by Wednesday afternoon, the employees will realise that with only two days left, one of which has already been ruled out, out the fire drill must be held on the following day. Again, no surprise, so it can't be Thursday either.
Once Thursday and Friday are ruled out, so is Wednesday. Because if it hasn't happened by Tuesday, the employees will realise that with only three days left (Wed-Fri), two of which have already been ruled out (Thurs and Fri) the fire drill must be expected on the following day - Wednesday. Again, if they're expecting it there's no surprise, so it can't be held on Wednesday.
You see my problem. If Wednesday is out, so Tuesday, and if Tuesday is out, so is Monday. On what day should I hold the fire drill? Surely it is impossible to surprise them? (My assistant unhelpfully suggested that I should just pick any day and then surprise the employees with an actual fire).