system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are Flies so good at getting into a home through a window, but incapable at getting outside through the same or multiple windows?
gpt
Because of the different senses they are using to get in and out.

Flies get into houses because they are searching for food.  Flies find food by smell.  Smell diffuses through air.  So imagine a fly trying to get into your house.  If it comes to a closed window, it can't smell food through the window so it ignores it.  If there's an open crack at the bottom of the window or an open door or something, it can smell the food and will follow the scent through the opening, which will actually let it get inside, because usually where the smell is going out there is an actual passage for the fly going in.  The exception here is screen doors and windows, and sometimes you _will_ see flies buzzing all around those trying to get inside and failing, because they can't follow the smell.  

Flies bump into glass windows but can't escape because flies, when not following a scent towards food, are attracted to light.  Flies aren't very smart, so they usually just home in on the brightest light source.  Inside a house, the light source is usually the window.  The fly doesn't really understand glass, so it keeps banging against it trying to fly toward the light.  It's not smart enough to realize it isn't making progress and search around for another way out.

To sum up: Flies following the light to leave a house tend to run into an impassible barrier (glass) while flies following smells to get into a house tend to find actual openings (open cracks, open doors).  So they get in more easily than they get out.