Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.

Question: What is a layer of saturated porous rock? Answer: an aquifer
An aquifer is a layer of saturated porous rock. It lies below the water table. An impermeable layer, such as clay, is below the aquifer.

Question: What phenomenon is crowding out other species and making all the other causes of extinction worse? Answer: human overpopulation
Human overpopulation, which is crowding out other species. It also makes all the other causes of extinction worse.

Question: All chemical reactions require what to get started? Answer: activation energy
Some chemical reactions release energy, whereas other chemical reactions absorb energy. All chemical reactions require activation energy to get started.