system
Given the following post, respond as if you're commenting on Reddit:
human
How long is a second?

IIRC, for measurements like the kilogram we base it off the number of atoms of a specific element. How exactly do we come up with a uniform version of a second, since time is kind of more abstract than mass.
gpt
It was originally 1/86400 the length of a mean solar day, however it was redefined such that a certain microwave emitted by Cesium atoms has an exact frequency.

The kilogram however is just the mass of a brick in France.