Developments:

At a beach near the village of Kartong in south-western Gambia, you can buy a bonga fish direct from the fishermen for 1 dalasi (about 2p). By the time it reaches the village market, 3km away, its price has doubled.

A Gambian child selling oranges in the city of Serrekunda understood how the economics of adding value works. He was selling peeled oranges for 2.5 dalasis each from his barrow on a street corner. I wanted an unpeeled orange, and expressed surprise when he said it would also cost 2.5 dalasis. Neither of us spoke each other's language, but when I mimed the gesture of peeling the orange, and implied that the work involved should increase the product's price relative to the unpeeled fruit, he twigged immediately, and explained to a bemused-looking friend the reasoning behind my argument. He must have been about 7 years old. Despite his precocious intelligence, however (or perhaps because of it), he still wouldn't drop his price for the unpeeled orange.