In this task, you are given a fact statement and question based on the fact. Your task is to generate the correct answer for the question. The answer needs to be generated using the context fact statement.

Fact: as the smoothness of something increases , the friction of that something will decrease when its surface moves against another surface. Question: A bobsled moves much faster as it leaves the concrete an touches the?
ice

Fact: uneven heating causes convection. Question: Convection is caused by uneven what?
warmness distribution

Fact: an electric car uses less gasoline than a regular car. Question: A battery operated vehicle requires everything but?

petroleum