The changing in price of shares are down to the number of people buying or selling stock. So, if there is a large demand for a stock then the share price will increase, and if there are lots of people selling stock, the share price will decrease.
How then, can the price at the open of one market day be different to the price at the close of the previous market day?
Surely if trading is closed between 16:30 and 08:00 then people can neither buy nor sell shares, and hence the share price cannot change. So how then does a share price change overnight?
Is it a case of such a large volume of buys/sells as soon as the market opens, that the time taken for a change in price is negligible, so the open price would appear to be different from the previous close?
Take for example a share price I was just looking at: Barrick Gold closed at 7.07 on 23 July and opened at 6.96 on 24 July.