Puzzle is professor's John Trono of St. Michael’s College in Vermont "Santa's Problem":
Santa Claus sleeps in his shop at the North Pole and can only be awakened in 2 cases:If all nine are back from their vacation in the South Pacific.
Some of the elves are having trouble making toys.
To allow Santa to get some sleep, elves can only wake him up when they are in a group of three. When three elves are having their problems solved, any other elves wishing to visit Santa must wait for those elves to return. If Santa wakes up to find three elves waiting at his shop’s door, along with the last reindeer having come back from the tropics, Santa has decided that the elves can wait until after Christmas, because it is more important to get his sleigh ready. (It is assumed that the reindeer do not want to leave the tropics, and therefore they stay there until the last moment possible.) The last reindeer to arrive must get Santa while the others wait in a warming hut before being harnessed to the sleigh.