The perfect explanation

Zhao Yu knew very well that as the King of Assassins, Liang perseverance never fought a battle he wasn't confident in. Since he decided to exchange the hostages, he must have made a detailed plan, so he definitely wouldn't be caught by the police so easily.

In order to avoid the worst case scenario, Zhao Yu went to Gao Facai and asked him to design two identical boxes for the skulls.

One of the boxes contained a skull. The other one was filled with a strong anesthetic. Once triggered, the anesthetic would spray out and cause a person to faint!

Forensic Gao was a top expert, and the anesthesia he produced was world-class. Even if it didn't hit the face, it would still have a certain anesthetic effect.

In addition, just in case, the two boxes were also equipped with tracking devices.

But, no matter what, the hidden weapons in the box were just Zhao Yu's backup plan.