21.   The water cannot be absorbed by the cells because any water that is taken up contains toxic sodium ions, which must be pushed out of the cell.

22.   This inside-outside difference is achieved by pumps in the cell membrane that work like submicroscopic waterwheels, exchanging sodium ions for potassium ions.

23.   When the neuron is stimulated, sodium ions rush into the cell and potassium ions rush out, leaving the neuron with a positive electrochemical charge.

24.   When jolted by an electric discharge from a neighboring cell, certain ion channels in the cell membrane open, allowing sodium ions to pour in.