How is energy transformed into thermal energy?

In today's science class students learned that all energy can be categorized as potential or kinetic.  The class jumped in surprise when Mrs. Woods stood on a chair and dropped a book!  In another demonstration there were two cups of sand, one that was almost full and one that was about one third full.  After taking their temperatures, each cup was shaken for five minutes.  Students hypothesized on what they thought would happen.  Some were pleased to see their predictions made scientific sense: the temperature of the sand in the cup that was only one third full increased more than the other because the sand had more room to move.  Students learned that motion can be transferred to thermal energy (heat).  Indeed, they all said they feel hot and sweaty after gym!