meiosis
Imagine shuffling two decks of cards together and then dealing out half-sized hands, where each hand is unique. Meiosis is the special kind of cell division that makes reproductive cells (eggs and sperm) by halving the chromosome number and reshuffling the genetic material, so no two gametes are quite alike.
In an organism with paired chromosomes (diploid), meiosis takes one cell through a single round of DNA copying followed by two rounds of division, called meiosis I and meiosis II. The result is four cells, each with only one copy of each chromosome (haploid). When two gametes later fuse at fertilization, the normal paired number is restored.
Two events during meiosis create the variety: homologous chromosomes pair up and swap segments (crossing over), and the pairs line up and separate independently of one another. Together these produce vast numbers of possible chromosome combinations, which is a major source of the genetic differences among offspring of the same parents.
A human cell with 46 chromosomes (23 pairs) undergoes meiosis to form sperm or eggs that each carry 23 chromosomes; fertilization then restores 46 in the new cell.
Halving and recombining: from 46 to 23 and back.
Do not confuse meiosis with mitosis. Mitosis makes two identical body cells for growth and repair; meiosis makes four genetically varied gametes and halves the chromosome number. The reduction step happens in meiosis I, when homologous pairs (not sister chromatids) separate.