What is the "hot" and "cold" category that Chinese culture separate their food into?
Per AsianResearch.org's article The Ancient Theory Behind Chinese Food:


  Generally
  speaking, foods that have a higher water content are considered cool,
  or yin, in nature. These are often foods that are boiled or steamed.
  Foods that have a higher energy content, particularly from fat, are
  considered warm, or yang, in nature. These are often foods that are
  fried or roasted.
  
  [...] 
  
  From [Chinese traditional element theory] comes the idea of the five flavours −
  bitter, sweet, spicy, salty and sour.
  
  These flavours are subdivided into yin and yang. Sweet and spicy foods
  are considered to be yin, while bitter, sour and salty foods are
  considered to be yang. The five elements also correspond with the
  colours red, yellow, white, blue and green, which are all considered
  when choosing ingredients.