Temperature is one of the most fundamental properties of planetary matter, as is evident from everyday experience such as the weather and cooking a meal, as well as from the most basic concepts of chemistry and thermodynamics. For example, H2O is a liquid between 273 K and 373 K (at standard pressure), a gas at higher temperatures, and a solid when it is colder; silicates undergo similar transitions at substantially higher temperatures and methane condenses and freezes at lower temperatures. Most substances expand when heated, with gases increasing in volume the most; the thermal expansion of liquid mercury allowed it to be the ‘active ingredient’ in most thermometers from the seventeenth century through the twentieth century. The equilibrium molecular composition of a given mixture of atoms often depends on temperature (as well as on pressure), and the time required for a mixture to reach chemical equilibrium generally decreases rapidly as temperature increases. Gradients in temperature and pressure are responsible for atmospheric winds (and, on Earth, ocean currents) as well as convective motions that can mix fluid material within planetary atmospheres and interiors. Earth's solid crust is dragged along by convective currents in the mantle, leading to continental drift. Temperature can even affect the orbital trajectory and rotation state of a body, as we have seen in our discussions of the Yarkovsky and YORP effects (§2.7.3 and §2.7.4, respectively).