Just to add a word of warning, the procedure above (linking cells by a =Sheet1! style reference) doesn't copy the data from the cell in Sheet1 to the cell in Sheet2 but establishes a constantly-updating link between the cells.
That means every time you open Sheet2, it opens Sheet1 and updates itself according to the current values of Sheet1.
That means, if you mangle Sheet1 and somehow replace a bunch of historical data with zeroes, those changes will be copied into Sheet2 the next time you open it.