In 1973 sociologist Mark Granovetter showed how the loose acquaintances, or "weak ties", in our social network exert a disproportionate influence over our behaviour and choices.
For a ball rolling across the floor, that outside force could be the friction between the ball and the floor, or it could be the toddler that kicks the ball in another direction.
Vitamin D made by the action of sunlight on the skin accounts for 90 per cent of the body's supply, but lower levels can also be obtained through food such as fish, eggs and breakfast cereals.
But what did for both was imperial overstretch, a rotten economy and, like Russia's today, a mostly unaccountable ruling caste that led a proud country to disaster.
An automobile produces enough heat to warm an average house in winter, and if a house were perfectly insulated, one adult could also produce more than enough heat to warm it.