Why do humans have two sets of teeth? ?

can someone me with this question? i know the answer but i need more explanation

A deciduous set that is replaced by a permanent set. There are generally more teeth in the second set than the first, and they will progressively grow into the gums in the growing jaw, filling gaps left by the deciduous teeth.

Na+ loss through sweet plus the dilution has lowered na+ concentration outside of neurons, altering the na+ equilibrium potential. sodium exists more outside the cells than inside. when na+ is lost by sweat plus the dilution( because they drink a lot of water) changes the concentration outside the cell, forcing the water to enter the cell and attempt to balance the concentration inside/outside.the problem is that the abundance of water will cause the cells to swell. most cells can adapt, however, the brain cells can't that's why the runner has confusion and can't control the muscles.  if not treated and this occurs in less than 48 hours, it can be immediately fatal .