Coal. Bituminous coal is the most plentiful in the U.S.

0.00%. Scientists believe the Earth is 4.56 billion years old.

Groundwater. Groundwater is stored in layers called aquifers.

The Big Dipper. The big dipper is the third largest of the recognized constellations.

Continuity Tester. A continuity tester determines if an electrical path can be established between two points.

29. The full moon is bracketed by a waxing and a waning gibbous.