An abbot visits a monastery and says that at least one of the monks there has a deadly but non-contagious disease. The only symptom is a red dot that will appear on the following day on the forehead of those infected. As they will die an agonising death within months, he urges those infected to kill themselves as soon as they know. But they have sworn an oath not to communicate with each other and there are no mirrors and they meet only once a day, at lunchtime. One month later, the abbot returns to find that all the infected monks killed themselves on the same day. How did they know they were infected and why did they die on the same day?

OK what am I missing? this is one of those irritating mensa type problems and I cant see what the solution is. I wondered if it was something to do with bending down over water -maybe in a glass? at lunchtime.