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  As the total kinetic energy of the particles in an object increases, the object gets warmer. Heat flows from a warmer object to a cooler one.
  When Matter Gets Warmer
  When you take a baked potato from an oven, the potato has kinetic energy while it is being moved. After you put the potato down, it still has a form of kinetic energy inside it. The atoms that make up the potato are always in motion. Each moving atom has a certain amount of kinetic energy. Thermal energy is the total of all the kinetic and potential energy of the atoms in an object.
  When any form of matter gets warmer, the kinetic
energy of the atoms or molecules that make it up increases. The object’s thermal energy increases.
  Thermal Energy and Phase Changes
  On Earth, most matter exists as a solid, a liquid, or
a gas. A change in thermal energy can lead to a
change in phase.
  When the thermal energy of a substance
increases, its particles move faster. If the thermal
energy of a solid increases enough, it melts into
a liquid. The liquid state of a substance always
has a higher thermal energy than its solid state.
   
  This beaker contains liquid
water, ice, and a balloon
filled with air. All these
materials have a temperature
of 0°C.