Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: Sperm ( Figure 1.1), the male reproductive cells, are tiny. In fact, they are the smallest cells in the human body. What do you think a sperm cell looks like? Some people think that it looks like a tadpole. Do you agree? A sperm has three main parts: 1. The head of the sperm contains the nucleus. The nucleus holds the DNA of the cell. The head also contains enzymes that help the sperm break through the cell membrane of an egg. 2. The midpiece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move. 3. The tail of the sperm moves like a propeller, around and around. This tail is a long flagella that pushes the sperm forward. A sperm can travel about 30 inches per hour. This may not sound very fast, but dont forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly. This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it swim? To make sperm, cells start in the testes and end in the epididymis. It takes up to two months to make sperm. The steps are explained below: 1. Special cells in the testes go through mitosis (cell division) to make identical copies of themselves. 2. The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. The spermatids are immature and cannot move on their own. 3. The spermatids are transported from the testes to the epididymis. Involuntary muscular contraction moves the spermatids along. 4. In the epididymis, spermatids slowly grow older and mature. They grow a tail. They also lose some of the cytoplasm from the head. It is here that the spermatids mature, becoming sperm cells. 5. When sperm are mature, they can swim. The mature sperm are stored in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with liquids from these glands, forming semen. The semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm!
Question: where do spermatids become sperm cells?
in the epididymis