Consider a monatomic ideal gas surrounded by an adiabatic, rigid, and impermeable wall. The system is therefore isolated and its internal energy conserved. In this case, it is common to consider the internal energy simply as the sum of the kinetic energy of all molecules. But can this amount really be kept at all times? What happens when a molecule changes direction following contact with a wall? After all, if it changes direction, there must be a brief moment when its kinetic energy is zero, and therefore the total kinetic energy of the system must decrease. Is there a contradiction with the conservation of internal energy? Explain in a few sentences.