it was not long before it was discovered by kids. instead of looting it, the kids mostly six- to twelve year old began playing with it. within a day or two, they had learned and taught one another to drag, create files and olders, perform other tasks. in three months they created more than a thousand folders.

윗 문장에서 과거완료를 사용한 이유가 뭘까요? 부탁드립니다.