Several traditions have developed in the USA around "soloing", including drenching the student with water and cutting off and permanently displaying the back of his or her shirt.
In American aviation lore, the traditional removal of a new pilot's shirt tail is a sign of the instructor's new confidence in his student after successful completion of the 1st solo flight. In the days of tandem trainers, the student sat in the front seat, with the instructor behind. As there were often no radios in these early days of aviation, the instructor would tug on the student pilot's shirttail to get his attention, and then yell in his ear. A successful first solo flight is an indication that the student can fly without the instructor ("instructor-less" flight). Hence, there is no longer a need for the shirt tail, and it is cut off by the (often) proud instructor, and sometimes displayed as a trophy.
November 17, 2011. Rene Suprise does his first solo flight at Stinson Airport! Congrats from all of us at Alamo.