Why do boys in Afghanistan during the winter of 1975 have gashes on their fingers?The Kite Runner book

In the book "The Kite Runner" the first part and most of the story takes place in Afghanistan.  The winter in Kabul is a time of school shutting down for several months due to the icy season.  The winter months also brings a time for kite flying.  Every year there would be a kite flying tournament.

To prepare for the tournament meant that the children would buy supplies and build their own kite.  They would work on the kites and pull the strings across their fingers as they set up the line.  The result was scars across their fingers caused by the line.

The gashes were horizontal lies cut across their fingers from a whole winter of practice flying the kites.    The children would gather together and inspect each others gashes and scars when they returned to school.

Page 50 has this part of the story.