汗牛充棟
Books which make oxen sweat, and which touch ceilings.
... 蔵書がきわめて多いことの形容。本が非常に多くて、牛車に積んで運ぶと牛も汗をかき、家の中に積み上げれば棟木にまで届いてしまう意から。
A figure of speech used to describe one's possession of an excessively large number of books.
The saying comes from the idea that the weight of such books, when loaded onto an oxcart, would cause the beast of burden to perspire; while its stacked height, when placed inside a house, would reach and touch its upper ridge beams (i.e. the ceiling).
Surprisingly, oxen, sheeps and goats can and do sweat.