My boss Prof. Li gave me some questions recently. One of them is whether there is entanglement among the macro-systems. We definitely know that there would be entanglement between the quantum systems, which are micro-scaled. When the system we discussed is too large to be viewed as micro-system, is there entanglement in this system? My boss enlightened me that firstly we should discuss the quantum system, for example, two spins being entangled by a electromagnative wave. We calculate the entanglement between them. Then we add the spin to the system, and calculate the entanglement again. Finally we get the relationship between the entanglement and the number of spins. When the number of spins apoaches to infinity, the micro-system becomes the macro-systems. So we solve the question at the beginning of this entry.