Why does metal expand?
Metal usually expands when heated because the atoms of
the metal start to move faster when they have more energy.
It's sort of like handing a piece of chocolate to a
5 year old; after they eat the chocolate, he or she starts
running around and bouncing off the walls, right?
The chocolate gives him/her more energy, so he/she moves
around more and explores more space. Same with the metal atoms;
when you heat the metal you give energy to the atoms and
they move around more as a result.
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Update: June 2012