For the fifth day of Christmas, we have quite a mysterious package that arrived from Switzerland.

...no, I think it was from Germany.

It wasn't quite such a surprise as you might think,

because when I turned it over -- of course it had a terrific wax seal,

nowadays people don't normally use wax seals --

but it had a Customs Declaration, and it said on it

"Guitar Plectrum."

So I knew what was inside, and it was far more mysterious than if I didn't.

And inside is a plectrum for plucking a guitar.

(You can see I don't know how to pluck a guitar)

It was made specially for me by Patrick Hufschmid.

Now what's interesting about it is that it contains

strontium illuminate.

Not pure strontium illuminate, but strontium illuminate

doped with a rare earth element.

Probably europium, though he didn't tell me which one.

So it glows in the dark! It's nice and shiny,

and it feels like plastic, though it's a slightly strange colour.

It could almost be ivory, but it's a bit too green.

It contains europium, and when you shine light on it

the europium stores some of the absorbed light energy

and then lets it out over a period of time.

Strontium Illuminate itself, that's a compound of

strontium, aluminium, and oxygen,

is not particularly special when it comes to optical properties.

But if you put in a rare earth -- for example europium

or sometimes people put in dysprosium,

so that's why I'm not certain, because he didn't say which one --

then the atoms of the rare earth -- which are isolated

rather like the fruit in a Christmas cake --

they absorb ultraviolet light, which you can't really see,

or short wavelength light, and then emit it in the visible light.

So the material glows in the dark

but it emits light much more slowly than it absorbs it

so it will go on glowing in the dark for several minutes

after you've illuminated it.

I'm not quite sure why one would need a glowing plectrum;

after all, if you're looking at the strings to see where you're plucking

you're probably not playing the guitar properly!

But it is really fun, and if I look at it in my hand, I can see it glowing.

So for the Fifth Day of Christmas, we have a glowing guitar plectrum,

four drinking receptacles,

three chemical badges,

two Periodic Table bedcovers,

and one piece of tartaric acid from a Swiss wine barrel.

Well it's sort of got to two now, so I'll put it back to make one.

(Brady) It's another brilliant act of defiance from Einstein towards that Nazi regime, isn't it? (laughter)

(Keith) Absolutely right

(Brady) Tell me what you think of this as a painting, you look at more paintings than me, I mean...

(Keith) I really enjoy it: Liebermann painted Einstein more than once

This is quite rough and sketchy, but it's lively you know

Einstein looks rather pleased with himself, quite happy...