Why do flies seem to come out of nowhere?

The full narration of the video.

An old saying goes: leave meat to rot, and maggots will grow out of it on their own. The kitchen bin seems to work the same way: lift the lid, and flies just pop out. People believed in "rotten meat makes maggots" for nearly two thousand years.

In 1668, an Italian doctor named Redi didn't buy it. He took a few jars and put a piece of meat in each. Half he left open. The other half he covered with gauze: air could get in, flies couldn't.

A few days later, the meat in the open jars was crawling with maggots. The covered meat rotted just the same, but not a single maggot was in it. Drawn by the smell, flies landed on the gauze, and the maggots showed up there instead.

Maggots don't come from meat. They hatch from eggs that flies lay. So where do the flies in your home come from? The moment you see one is like a parcel at the door. But that order was placed a week ago.

The one placing it is a mother fly. She finds leftovers in the bin and lays about a hundred eggs at once. Over her life, she can lay around five hundred. In less than a day, the eggs hatch into little white maggots that burrow into the scraps and eat.

After a few days of eating, they seal themselves in a hard shell. Inside, they don't eat or move. They slowly change. A few days later, the shell pops open, and out crawls a fly that can fly. In hot weather, egg to fly takes as little as seven to ten days.

At 21 degrees Celsius, it takes about three weeks. All that time, they were hiding at the bottom of the bag, inside the scraps, and you never saw a thing. And the hundred or so from one order arrive within a day or two of each other.

So you don't see one fly. You see several, all at once. It looks like they came out of nowhere. Those tiny bugs around the fruit bowl are the same story. They're fruit flies, smaller than a grain of rice.

Sometimes their eggs were laid on ripe fruit back at the store, and came home with you. Gunk in the drain, a damp mop, a dirty dishcloth: any of these can raise a whole batch. Flies never come from nowhere. They just spend their first week where you can't see.

In summer, take the trash out every two or three days, and put ripe fruit in the fridge. That order never gets delivered. In your home, where do flies show up first: the trash bin, or the fruit bowl?