What are the boys in the square doing in the story "The Lottery"?

It is the morning of the lottery.  This event happened every year in every town on June 27th unless it was a huge town, and they needed two days for the event.  School was just recently out for the summer, and the young boys were the first to enter the square. The boys of the town gathered quietly at first and then,

“…..broke into boisterous play.”  (pg 1)

Bobby Martin was the first boy to fill his pockets with stones, and the other boys then followed his lead. They chose,

“…..the smoothest and roundest stones.” (pg 1)

It is obvious that they are choosing these stones for a specific purpose.  Bobby, Harry Jones, and Dickie Delacrois made a huge pile a stones in one corner of the square.  They guarded that pile because other boys would try to take them.

This foreshadows what is to come later.  The reader knows that the boys must be preparing for something that will happen later in the book, otherwise, why bother collecting and piling the stones.