Crime scene: 162 mystery solving puzzles to find out who the murderer is

Chapter 77 The Needle In The Egg
One day, the big detective G·K was drinking in a bar, and suddenly heard a quarrel at a table, and then a person stood up and wanted to perform egg swallowing, which may have been purely caused by a bet after drinking, and everyone kept agitating him, He had no choice but to bite the bullet, but he didn't know that one of them wanted to take the opportunity to kill him.

I saw that person opened the first egg, raised his head and swallowed it with a "gulu", and then swallowed two more.The whole audience burst into applause, and G·K also watched with relish.

When he swallowed the fourth egg, his face suddenly changed, and then he vomited blood, unable to speak, everyone present was shocked, G·K immediately called an ambulance and sent him to the hospital.

After he left, G.K launched an investigation and found that there was a steel needle with two pointed ends in the fourth egg, and found that all the eggs were provided by a man named Mao San, so G.K found him , but he denied it, so G·K revealed the technique he used!
Smart people, how did Mao San penetrate the egg without leaving any traces?

[Answer] Soak the egg in vinegar, prick the needle into it after it becomes soft, and then put it in water until it hardens.

(End of this chapter)