老师对同学说："Wow, you are on fire!"，同学吓一跳，以为自己身上着火了。
on fire的确有"着火"的意思，比如：
The house is on fire.
房子起火了！
 
但是on fire可不仅仅有“着火”的意思，它还可以表示“情绪饱满、热情高涨、高水平发挥”，比如：
Man, you are on fire!
哥们儿，你状态真不错！
You guys were on fire in the last quarter.
你们最后一节真是打得太棒了。
 
“上火”英语怎么说？
 
本来“上火”这个概念对许多外国友人来说已经很深奥了，可别说成“I'm on fire”让别人更摸不着头脑。
究竟怎么才能准确表达“上火”呢?
英语中有这样一个词：heatiness
a characteristic of certain foods or stimulants said to cause emotional or physical reactions associated with temper, fever, passion, excess, or true heat.（From Chinese culture and medicine.）
一种食物或刺激物的特性，据说能引起情绪或身体上的反应，与脾气、发烧、情绪、过量或热有关。（源自中医）
 
heatiness的形容词形式是heaty：
You're heaty.
你火大（上火）。
 
这个词其实算是中医术语，在此基础上我们还可以对外国友人解释一下这个中医概念：
"Shanghuo(heatiness)" is a unique concept in traditional Chinese medicine.
“上火”是传统中医中一个特有的概念。
 
Traditional Chinese medicine categorises food as凉lèung (“cooling”),热气yiht hei (“warming”) and neutral–and practitioners use the nature of the foods to balance the body's yin and yang.
传统中医把食物分为凉性、热性和中性——医生们则用这些食物的特性平衡人体的阴阳。
 
TCM describes heaty foods as having the ability to warm and improve circulation, dispel cold and stimulate the body. When these foods are taken in excess (yang is greater than yin, your body will generate too much heat), heatiness is caused, resulting in symptoms such as fever, sore throat, mouth ulcers, acne, excessive thirst, redness of the skin and irritability.
中医认为，热性食物具有保暖、促进血液循环、驱寒和激发身体的功能。当这些食物摄入过量（体内的阳压过阴时，你的身体会产生过多的热气），会导致“上火”的现象，引发发烧、喉咙痛、口腔溃疡、痤疮、极度口渴、皮肤发红和易怒等症状。