Here's a breakdown:

* Meiosis: This is a type of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes as the parent cell.

* Crossing over: During meiosis, homologous chromosomes (pairs of chromosomes with the same genes) line up next to each other. At this point, they can exchange segments of DNA. This exchange creates new combinations of alleles (versions of genes) on the chromosomes, increasing genetic diversity.

Key points about crossing over:

* It occurs during prophase I of meiosis.

* It contributes to genetic variation among offspring.

* It is a major source of genetic diversity in sexually reproducing organisms.