> > > > > > Twenty persons sat down for dinner.
> > > > > > Twenty-one left the dining table.
> > > > > > How come?
>
> I say the cake served for dessert contained one of those dancers who pop out, and therefore, after everyone enjoyed a good laugh at the birthday boy's expense, there were twenty-one people to leave the table.

Or, twenty-one people sat down at the dining table. One of them sat down to have a drink and not to have dinner, and they all eventually left.

-Morris Cecil "Morris Cecil Glalet" Glalet, Tue 13 Jul A.D. 1999