7 year old Philadelphia girl shot by 8 year old brother

In the second similar incident in almost as many days, a 7 year old Philadelphia girl was shot and injured by her 8 year old brother. From an article:

Police say an eight-year-old girl was shot by her seven-year-old brother while the two were allegedly playing with a gun they found inside their Philadelphia home.

The incident happened in the 1900 block of Georgian Street in West Oak Lane around 10 a.m. on Thursday. The injured child, who was taken to the Einstein Hospital by family members with a gunshot wound to her arm, was transferred to Children’s Hospital of Philadelphia where she is listed in stable condition.

Police say the gun was kept disassembled in a locked case and it appears the little boy grabbed the keys and assembled the weapon before it discharged.

Children seem to know where guns are in homes. How did the boy learn to assemble a gun?

Every gun in the hands of a child must first pass through the hands of an adult.