system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What about a glass ketchup bottle makes it so difficult to easily pour?
gpt
The ketchup is thick enough to create an air tight seal at the neck of the bottle.

In order for ketchup to leave the bottle there needs to be airflow to replace the ketchup.

Unless the bottle is plastic so you can squeeze and reduce the volume of the container without needing air to replace the ketchup.