Why is Earth’s inner core solid instead of liquid?

We usually think of materials like iron and nickel melting as the temperature gets higher, and Earth’s inner core is hotter than its outer core. Why, then, is the inner core solid while the outer core is liquid? The answer has to do with pressure. Both temperature and pressure increase with depth in a planet. While higher temperature tends to make metals like iron and nickel melt, higher pressure tends to keep them solid. So even though the inner core is hotter than the outer core, the higher pressure keeps the inner core solid while the outer core is liquid.