why factory building are example of capitial goods?

Because the term 'CAPITAL GOODS' refers to REAL OBJECTS owned individuals, organizations or goverment to be used IN THE PRODUCTION of other goods or commodities.
As factories and machinery can both be used to produce goods....of course they are considered capital goods!

2008-09-20 15:05:57 補充：
OPPS i missed your last sentence....i'll explain it again in Chinese
Capital Goods 是指可用於貨物生產或者involve in 生產過程中的實際物體。
由於廠房和機器都是用於生產貨物、產品的，它們就是capital goods。