This is not sake but "shouchuu" (or shochu) Japanese vodka, made from potatoes, wheat, or rice and distilled to a fairly potent 25 percent alcohol by volume. It is rumoured not to give the drinker much of a hangover due to its purity. It is drunk straight, with ice, or with hot water (for maximum effect).
The Japanese are not ashamed to say that they drink alcohol for its effect. The bottles are big, and the consumption among middle aged men is quite high.
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