1 Caption: King's College London, 1860:
1 James Clerk Maxwell: Oh, this is too difficult. I cannot get these equations governing electro-magnetism to come out right.
2 Maidservant: Oh, Professor Maxwell, sir. You work too hard. Have some of my Christmas pudding.
3 James Clerk Maxwell: This is delicious. How do you achieve such a light texture?
4 Maidservant: I add extra dried fruit to take the place of some of the suet. I call it the displacement currants.

I could write a long and technical explanation of this joke...

But then I realised I already have.