Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: The male gametophyte produces flagellated sperm that must swim to the egg formed by the female gametophyte. For this reason, sexual reproduction must happen in the presence of water. Therefore, nonvascular plants tend to live in moist environments. Though the life of a nonvascular seedless plant is a cycle, this can be considered the initial step in the life cycle. Answer: water
Output:
When flagellated sperm must swim to the egg, sexual reproduction requires the presence of what substance?