I found a sandwich that my son hadn't eaten after a week in his bag. It was covered in maggots. How did they get in the baggie?
The maggots were either always there, or the bag was open at some point
No one knows, it's like they're magic or something
The maggots were attracted to the decaying sandwich. Maggots will usually thrive on stale food pieces including meat and bread. They are known to be responsive and would eat anything. Learn more about these at http://www.buzzle.com/articles/maggots-life-cycle.html.
flies don't need much of an opening to get in, and with the summer heat it doesn't take long for their eggs to hatch.
They hatch from fly eggs. One fly had to come in contact with one of the sandwich ingredients at some point and leave some eggs. That's really all it takes.
At some point he may have left the sandwich sit just long enough for a damn fly to do its deed. At least the bag was sealed. Ahh those kids. I've already found crayfish and toads stuffed in pockets and forgotten until laundry day.