Given a scientific question, generate a correct answer to it.

Example Input: What do many fungi secrete that break down complex molecules to smaller molecules that can be absorbed?
Example Output: enzymes

Example Input: What often relies on cooperation between the motor and autonomic nervous systems?
Example Output: homeostasis

Example Input: When a solid substance is involved in a chemical reaction, only the matter from which part of the solid is exposed to other reactants?
Example Output:
the surface