Only certain planets (like ours) actually have a liquid core with a solid outer crust. Most rocky planets are like Mars, completely solid all the way through.

I believe what the other commenter was getting at is that, on large scales, the solids that make up planets behave like liquids do at smaller scales. Since if you zoom out, there's not much different between a bunch of rocks loosely held together with gravity and a water droplet weakly held together by surface tension.

[–] Womble@piefed.world 4 points 4 hours ago

Even if they are "solid" at a human sized scale they are effectively liquid on a planetary scale. Gravity is just so much stronger than the internal stiffness at that scale that they behave as if they are a liquid with effectively no interal stiffness. That's why as you get smaller down to moons and asteroids you start to se shapes that arent spheres, the materials strength has sufficient strength to be able to resist gravity at those scales and the material is acting more like a solid.