Every year on the last day before Christmas vacation, I ask my students the question: could Santa Claus possibly survive his journey around the world to deliver Christmas gifts to children? I step up to the white board, draw a cartoon of Santa and his sleigh zooming around the world, and ask, "Tell me what you know about Santa's journey."

I try not to "lead" the discussion very much - every year students come up with different ideas about how to approach analyzing his journey. But here are some of the key things we discuss each year:

• What is the earth's population?
• How many families are there?
• Can we assume one family per house? What about apartment complexes?
• Does Santa visit all families? Or just ones with children? Or just ones with "nice" children?
• What is the average distance between houses? (This leads to questions about the size of the earth, and the amount of landmass, and the most efficient route to travel).
• Does Santa have to actually go down the chimney? Or can he just drop presents down the chimney and keep going?
• Can he stop once at an apartment bullding and drop down the chimney the presents for all 20 families living there (one of my students suggested this year that he could do this, and the landlord would be responsible for delivering presents to the different apartments!)? How does that affect the number of stops?
• How much time does he have to travel between each house?

Once we've come up with a figure for the time to travel between houses, and the distance traveled per house, I remind the students of gravity trains, which accelerate for half the journey and decelerate for the other half. To minimize Santa's acceleration, this is the route to go. So we treat Santa leaving one house as the beginning, and the halfway point as the end, and work through the kinematics to find his acceleration (and maximum velocity).

What we invariably find is that Santa's acceleration is so absurdly over the 5 g's that the human body can survive that he is clearly dead. In addition, we note that if he COULD survive, there would be a sonic boom at each location, which would wake everyone up. Students will occasionally point out that maybe Santa isn't "human" so he can survive those forces. At that point I remind them that Santa Claus, AKA Saint Nick, AKA Bishop Nicklaus, was a real live 3rd century Christian Bishop of the Turkish city of Myra, so he really was human (many students don't know that there was a real Saint Nick). At any rate, we often agree that there must be some form of "Christmas magic" protecting him.

When we're all done, I tell the students to save the details of this problem in their notes, because after we've studied work, energy and power, We can follow up on this problem and determine the horsepower of a flying reindeer.

This is a wonderful opportunity for students to practice estimation skills, but just as important for the last day before Christmas, it's a lot of fun. Every year this problem results in all sorts of funny discussions. This year, for example, the students observed that Santa wasn't really wearing a red suit - he was just on fire from the air friction.

Contemplating the demise of Santa Claus - it might seem morbid, but it's the perfect way to celebrate Christmas in a Physics class.