We are human and we have to eat food in varieties in order to keep our body in good functions. But, animal is different. They can live on a single food, such as that lions live on meat only, pandas live on bamboo and elephants live on green grass and tree leaves. All of them are very strong and stubborn. This proves that single food can also provide nutrition enough for animals to live.
For human, not only we need to eat more varieties, but, also, we wish to eat expensive food which, we think that once it is expensive, must be a quality of high value to our body. An example tells us that we all recognize the swallow nest to be best to our health since it is so expensive.
Thinking deeply that the effect of swallow nest has not been analyzed by the scientists so far, to prove of high value to human body. Its goodness is from the advertisement of swallow nest dealers who, in order to promote and sell at high price, have to acclaim that if you eat swallow nest, your face will be shinning and you can stay your youth longer. Such words are particularly acceptable to ladies.
Why the swallow nest is so expensive because it can only be obtained from the tops of mountain cave. Workers have to risk their lives to get it, which is the basic reason to be expensive.