As part of an ecumenical exchange, a rabbi is invited to an evening meal at a monastery.
After a delightful meal one of the monks stands up and says "281"
There are a few quiet giggles, then another stands up and says "356" Again more giggles.
Intrigued as to what is going on, the rabbi enquires of the abbot about what the numbers mean.
"Well" says the abbot, "we have been living together for so many years that we know all each others jokes, so to save having to repeat them, we have given each of them a number, its simply a way of saving time."
"Why don't you have a go?" suggests the abbot.
So the rabbi stands up and says "2829"
All the monks suddenly collapse in fits of unrestrained laughter. Finally the abbot restrains himself with great difficulty.
The rabbi asks, "Why is my joke so funny?"
The monk replies "They haven't heard that one before!"