B trusts A, C trusts B, D trusts C, and so on.

A tumbles over, and down go B, and C, and D.That's the whole of it, my dear.

Colonel Burr used to say that his rule was to keep himself just out of reach of any other block.

If they knock me over, my dear Miss Bunley, he once said to me--ah! May, what a voice he said it in, what an eye!--if they knock me over, I shall be so busy picking myself up that I shall be forced to be selfish, and can't help them, so I had better keep away, and then I can be of some service.

That was Colonel Burr's principle.