instruction:
Given a scientific passage and an answer, generate a question for the given answer.
question:
Passage: Sperm ( Figure below ), 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?. Answer: sperm
answer:
What are the male reproductive cells called?

question:
Passage: The heart is a muscular organ in the chest that consists mainly of cardiac muscle. It pumps blood through blood vessels by repeated, rhythmic contractions. Blood flows through four chambers of the heart when it beats. Heartbeat is controlled by electrical signals from special cells within the heart called the pacemaker. Answer: cardiac muscle
answer:
What type of muscle makes up most of the heart?

question:
Passage: Sound is a form of energy that travels in waves. Sound waves cannot travel through empty space, but they can travel through gases. Gases in the air allow us to hear most of the sounds in our world. Because of air, you can hear birds singing, horns tooting, and friends laughing. Without the atmosphere, the world would be a silent, eerie place. Answer: gases
answer:
What helps sound travel in waves?