system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are we not able to open car doors if completely submerged underwater?

I see this in movies a lot and don't really understand why this mechanism would be implemented in cars.
gpt
The weight of the water is what prevents you from opening the door, it's the equivalent of having several big fat guys leaning on your door from the outside. 

Once your car is filled with water the pressure is the same on both sides of the door and you can open it without issue.

Mythbusters has a whole episode about this.