Why must the number of chromosomes in the sperm and egg be half the number of chromosomes in the other cells of an organism?

when fertilization takes place, the half of the chromosomes from the egg combine with the half of the chromosomes from the sperm to form a full 23 pairs of chromosomes which contain the genes of the offspring.

answer; i believe that the correct answer is; ///bodies of water store heat during the day and release it at night;