Todd Krohn, age 15, of Casper, Wyo., for his question:

WHEN WAS THE WHEEL INVENTED?

We don't know exactly when the wheel was invented, but we are pretty sure it was discovered about 5,000 years ago. We are definitely

sure that it was one of the most important steps in mans development.

The first important use of the wheel was in transportation. Wheeled carts were certainly easier and faster to pull than sledges. They also permitted man to make better use of the work of horses and other animals.

Without the wheel, today's civilization would be impossible.

The earliest known wheel vehicles were shown on tablets made in Sumer about 3500 B.C. The people in this ancient Mesopotamian region built chariots to carry their armies. Ancient people also pull wheels on devices used for irrigation and pottery making.

After the wheel came the wheel and axle, a mechanical device used in lifting loads. The wheel and axle is a first class lever.