system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did scientists conclude/figure out that the Earth's inner core is solid while the outer core is molten?
gpt
The waves generated during earthquakes travel throughout the planet.  If you know when and where the quake was, you can calculate how long it should take the waves to get to your monitors on the other side of the planet.  Trouble was that the waves were not arriving when and where they were calculated assuming a total liquid core.   Further calculations showed that the wave paths were getting disturbed by something and that something was calculated to be a solid core.