TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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: Metabolism produces waste energy in the form of heat, which helps warm-blooded (endotherm) animals conserve and maintain heat at a relatively constant body temperature.

SOLUTION: Metabolism produces waste energy in the form of heat, which helps what type of animal conserve and maintain heat at a relatively constant body temperature?

PROBLEM: Atoms carry a neutral charge.

SOLUTION:
What charge do atoms carry?