system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are we good at determining how full a container is just by holding it?

Basically the title. When we pick up a sealed container with our hands, say, an oxygen cylinder, most of us can determine how filled it is. But how?
gpt
You can feel the center of gravity of an object, because simplified that is where your forced applies. It is basically the rotational axis of an object. You can test this by suspending something a disc in water and then weighing it down with coins on one side. It will always spin closer to the coins.

Now the center of gravity does not equal the fullness of the container, but we can approximate it, the heavier the substance, the easier. A container that is empty will have its normal center of gravity, but one that is half-full will have it shifted towards the filled half.