一位小帅哥微信我：我以前不知道food是“双性词”：又可数，又不可数。我，居然晕了：我从来不考虑这些“无聊的问题”。反正“随便”用，用错了，也许有人在乎；也许，老美根本不在乎。
但是，既然想当老师，不得不“现学现卖”。网上什么都有。在中国，不能上Google，我就上Bing。
The noun food is a member of the category mass (noncount) nouns. A mass noun represents an undifferentiated "mass" of something. Food is used as a mass noun in utterances like
例子：
I"ve never seen so much food in my life! Who"s going to eat it all?
I"ve left some food for the cat in the fridge. Be sure to heat it a little before you give it to her.
不过，food也可以作为可数名词。（所以，有时英语不好学。）
The noun food is not always a mass noun, however. It is often used as a count noun:
At this year"s international fair we sampled foods (kinds of food) from many more countries than last year.
上面的foods是指“各种各样的食物”。
事实上，不少不可数名词，可以作为可数名词。（我起床后立马写这篇博客，还没有喝咖啡。但是，室外1C的杭州，室内穿着羽绒服的我，还是非常清醒的。）
下面这三种用法，值得好好学习：
Many mass nouns in English can be used as count nouns. Quirk et al. state that noncount nouns may be converted to count nouns to express three ideas:
"1) "A unit of N": two coffees; two (huge) cheeses
原来，这就是为什么我的一位美国同事，常常说：I need just one coffee in the morning. 我，常常弱弱地说：I can only drink 1/3 tube of coffee. （因为，我喝咖啡是为了做瑜伽。微波炉热半杯牛奶，倒入1/3条咖啡粉。津津有味地喝完这“药”。）
2) "A kind of N": Some paints are more lasting than others; This is a better bread than the one I bought last.
我最怕用的詞包括bread，原来它也是“双性的”。
3) "An instance of N": A difficulty; small kindnesses; a miserable failure; home truths; a great injustice"*
我就不评论3）了，因为，罗马不是一天建成的。
*Quirk et al., A Comprehensive Grammar of the English Language (Longman, 1985), Appendix I.53
想继续学习，请看：
https://thegrammarexchange.infopop.cc/topic/food-vs-foods-1
如果你想提高科技英语写作能力，请跟我来...
http://blog.sciencenet.cn/blog-306792-1146577.html
