A sperm cell uses its flagellum, commonly referred to as a tail, to propel itself toward the female oocyte, or egg. The flagellum is the longest part of the 25μm of the cell and surprisingly requires only 12×1CT18W of power to propel it. Motile sperm cells typically move via flagella and require water in order to swim towards the egg for fertilization. These cells cannot swim backwards due to the nature of their propulsion. The uniflagellated sperm cells (with one flagellum) produced in most animals are referred to as spermatozoa and are known to vary in size.