Everyone has a funny turn when they try to draw a Christmas tree

Today we bring you the second half of this year's Melvyn Bragg Christmas Science Lecture, an annual institution which aims to bring scientific awareness to the public by bringing out the scientific principles on which Christmas is based. Professor Gene Jones is this year's speaker.

It is commonly believed that when the human body undergoes heat loss, most of it goes from the top of the head - in other words, that if we wear a hat, we prevent a lot of the heat from leaving the body. This is certainly true in most circumstances, but not so true when we are out for a long, muddy, Christmas Day walk wearing wellington boots. The presence of the feet in the boots creates an enclosed warm, damp microclimate which pushes a funnel of warm, damp air up the side of the legs and out into the atmosphere, and common sense tells us, never mind science, that this causes a tremendous amount of heat loss.

Yet although we are sensible enough to wear hats, we never have the sense to take the same precautions when it comes to gum boots. To prevent heat loss from gum boots, all you need do is stitch or glue some sort of lapping round the top of the boots, a bit like a pelmet, perhaps, which would act as a stopper to all that warmth escaping...

What's that, sir? It would look rather stupid? Of course it would, but all sensible things look stupid to begin with. Gum boots looked stupid to begin with. Baseball caps worn backwards look stupid. Actually, baseball caps worn the right way round look pretty stupid, especially in a country where baseball isn't played...

Now, you may get the impression from what I have been saying that science can explain everything, but of course this isn't so. And indeed there is one Christmas mystery that science has never explained, and that is to do with Christmas trees.

Here's a little experiment you can do at home. Get a child or a grown- up to do a drawing of a Christmas tree. We can all do that. Now, take the drawing and put it beside your own Christmas tree, and ask the artist if they can spot the difference. Usually, they can't. But the fact of the matter is that we always draw Christmas trees with the branches drooping down. And yet Christmas trees in real life have branches that stick up. We draw them sticking downwards and see them sticking upwards. We don't do it on purpose. We do it because we have been taught to do it that way and yet nobody, until it is pointed out to them, ever realises that the two things are opposite to each other.

Why is this ?

How come we can be taught to do something which is the direct opposite of our experience?

Is there some genetic flaw in our visual make-up?

Is there a human predisposition to certain ways of drawing the world?

No - it's because we're stupid and lazy.

We also know that Christmas trees are highly inflammable, and yet for many years we insisted on putting candles on the tree branches. No doubt there were many fires and many houses burned down, yet it was not until the invention of electric light that candles were phased out, even though there is always one dud bulb in any set of Christmas lights.

Does this mean that there was always one dud candle on every Christmas tree in the old days?

We simply don't know. But it's a pleasing thought.

Incidentally, when it come to the time to make a bonfire after Christmas of all the wrappings and the tree and boxes, remember that Fulbright's November 5th Law operates here as well. Fulbright's Law of November 5th states that when you put all the dead fireworks on a bonfire on 6 November, there will always be enough explosive left to make a few bangs. By extension, when you put the Christmas rubbish on the fire, there will always be a few gifts still in there. So go through the paper first!

And finally, the big question: can reindeer really fly? The scientific answer to this is, of course; no they can't. But the question a scientist would like to ask is this: why do people THINK that reindeer can fly? And even better, why do people think that when reindeer fly, they move their legs and gallop in mid-air, even though this wouldn't help them to fly at all?

(And who was it once pointed out that when Superman flies through the air, there is no need for him to be in a diving position - he could simply sit in mid-air like a first-class airline passenger, reading a magazine?)

I hope to answer these and many more questions in next year's Melvyn Bragg Christmas Science Lecture.