Given a scientific passage and an answer, generate a question for the given answer.

Input: Consider Input: Passage: You can tell that this sponge is saturated with water because when it is squeezed, the water pours out. When something is saturated with water, it already holds as much water as possible. Compounds called hydrocarbons, which contain only carbon and hydrogen, can also be saturated, but not with water. Answer: hydrocarbons

Output: What compounds contain only carbon and hydrogen?

Input: Consider Input: Passage: Imagine walking in the forest in Figure below . The towering trees have been growing here for hundreds of years. It may seem as though the forest has been there forever. But no ecosystem is truly static. The numbers and types of species in most ecosystems change to some degree through time. This is called ecological succession. Important cases of ecological succession are primary succession and secondary succession. Answer: ecological succession

Output: The numbers and types of species in most ecosystems change to some degree through time and this is called?

Input: Consider Input: Passage: One force acting on youand all the other objects on Earthis gravity. Look at the physics book in the Figure below . Gravity pulls the book downward with a force of 20 Newtons. Why doesn't the book fall to the ground? The table pushes upward on the book with the same amount of force. The combined force, or net force , acting on the book is 0 Newtons. That's because upward and downward forces are balanced, so they cancel out. You can learn more about net force at this URL:. Answer: newtons
Output: The pull of gravity is expressed as a force of what?