What energy is often called the "graveyard of kinetic energy" and why?

Heat is often called the "graveyard of kinetic energy". Heat due to friction. Due to friction, kinetic energy is converted to heat and not potential energy; thus, the kinetic energy is "lost" since heat cannot easily be converted back into kinetic energy.

answers: to make sure that all of the broken pieces of tablet were added to the water, to allow for fair comparison of all three trials, with different particle sizes to avoid experimental error