Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called implantation . It usually happens about a week after fertilization. Now the implanted ball of cells is ready to continue its development into a baby boy or girl. Answer: rust
Output:
What is the process of oxidized iron that has been exposed to air and water?