Matt Kramer, age 13, of Springfield, Ill., for his question:

WHAT IS AN OCTANE?

An octane number is a rating that tells how much motor fuel "knocks." "Knocking" occurs when the last of the fuel in an engine cylinder burns. It causes some loss in engine power.

Two test fuels, normal heptane and isoctane, are blended together for tests to determine octane number. Normal heptane has an octane number of zero and isoctane has a value of 100.

Gasolines are then compared with test fuel blends to find one that produces the same "knock" as the test fuel. If a test blend has 85 percent is octane and 15 percent normal heptane, the gasoline is given an octane number of 85.

Most gasolines today have octane numbers of from 90 to 100.