Problem: 
Q: Q: What are a class of biochemical compounds that living things use to store energy?   A:
Answer: lipids
Problem: 
Q: Q: What do you call a species that has died out in the past?   A:
Answer: extinct
Problem: 
Q: Q: Unlike nuclear fission, which involves dangerous radioisotopes, nuclear fusion involves what two harmless elements?   A:
Answer:
hydrogen and helium