"Urea is a comparatively simple substance. Its molecule consists of only eight atoms which hold together quite firmly. But as a rule, the more atoms that go to make up a molecule, the weaker is the force that keeps them in position. An analogy is provided by a house of cards. The first and second stories of such a house are fairly firm, but as each successive card is added, the whole structure becomes a little more precarious until eventually the addition of one card or a slight jolt of the table is enough to bring the whole flimsy structure down. If one builds very carefully and takes precautions to prevent the table from being in the least shaken, one may succeed in constructing a high house. Some organic molecules are like a very high house of cards, but the strange thing is that they are like one built on a table that is persistently and violently jolted. Such molecules are shaken by the chemical influence of, and mechanical collisions with, the active neighbouring molecules."