TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: Surface tension is responsible for the fact that small insects can walk on water.

SOLUTION: Surface tension is responsible for the fact that small insects can do this on water?

PROBLEM: A concave lens is thicker at the edges than it is in the middle.

SOLUTION: What type of lens is thicker at the edges than it is in the middle?

PROBLEM: Ash that enters the air naturally as a result of a volcano eruption is classified as a primary pollutant.

SOLUTION:
Ash that enters the air naturally as a result of a volcano eruption is classified as what kind of pollutant?