When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases. The motion and spacing of the particles determines the state of matter of the substance. The end result of increased molecular motion is that the object expands and takes up more space.
Hot
Girl: "Helia is literally the definition of hot"
Guy: "The dude from that fairy show??"
Girl 2: "Yup!!"
Yea, there you go, Nicole Scherzinger is hot... what else?