Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.
Example: Paragraph- Sent 1: Obama was born on August 4, 1961, at Kapiʻolani Maternity & Gynecological Hospital in Honolulu, Hawaii.
Sent 2: He is the only President to have been born in Hawaii.
Sent 3: He was born to a white mother and a black father.
Sent 4: His mother, Ann Dunham (1942-1995), was born in Wichita, Kansas, of mostly English descent, with some German, Irish, Scottish, Swiss, and Welsh ancestry. Question: How old was Obama's mother when he was born?
Example solution: 19.
Example explanation: Obama was born in 1961. His mother was born in 1942 and 1961-1942=19. Therefore, any of the following responses are considred correct: '19' and 'almost twenty'.

Problem: Paragraph- Sent 1: If you build a campfire, you start with a large stack of sticks and logs.
Sent 2: As the fire burns, the stack slowly shrinks.
Sent 3: By the end of the evening, all that is left is a small pile of ashes.
Sent 4: What happened to the matter that you started with?
Sent 5: Was it destroyed by the flames?
Sent 6: It may seem that way.
Sent 7: What do you think happened?
Sent 8: The truth is that the same amount of matter still exists.
Sent 9: The wood changed not only to ashes, but also to carbon dioxide, water vapor, and other gases.
Sent 10: The gases floated off into the air, leaving behind just the ashes.
Sent 11: Although matter was changed, it was not created or destroyed.
Sent 12: Because the same amount of matter still exists, we can say that matter is conserved.
Sent 13: You may wonder how it can be conserved if something is now missing?
Sent 14: Assume you had measured the mass of the wood before you burned it.
Sent 15: Assume you had also trapped the gases released by the burning wood and measured their mass and the mass of the ashes.
Sent 16: What would you find?
Sent 17: The ashes and gases combined have the same mass as the wood you started with.
Sent 18: This example illustrates the law of conservation of mass. 
Question: As a fire burns, what stack slowly shrinks?.

Solution: The stack of sticks and logs.