You will be given a definition of a task first, then some input of the task.
Given a scientific passage and an answer, generate a question for the given answer.

Passage: A warm-air heating system uses thermal energy to heat air. It then forces the warm air through a system of ducts. You can see a diagram of this type of heating system in Figure below . Typically, the air is heated in a furnace that burns natural gas or heating oil. When the air is warm, a fan blows it through the ducts and out through vents that are located in each room. Warm air blowing out of a vent moves across the room, pushing cold air out of the way. The cold air enters an intake vent on the opposite side of the room and returns to the furnace with the help of another fan. In the furnace, the cold air is heated, and the cycle repeats. Answer: thermal
Output:
A warm-air heating system uses what kind of energy to heat air?