One such material is asphalt, a substance often placed on top of buildings and used to cover road surfaces.

VOA: special.2010.03.02

A BTU is the amount of heat energy needed to raise the temperature of about one-half kilogram of a substance by one degree Fahrenheit.

VOA: special.2010.11.23

He made everything - except coffee - from sourdough, a substance used in making sourdough bread.

VOA: special.2009.01.24

The female mosquito uses its thin sucking tube to break the skin, find blood and inject the victim with a substance that keeps blood flowing.

VOA: special.2009.05.26

Ultraviolet light from the sun changes a substance in the skin into vitamin D.

VOA: special.2009.04.21

Drop several seeds over the surface of the pot and cover with a thin layer of vermiculite, a substance that can hold air, water and nutrients.

VOA: special.2009.04.21

A substance that can produce deadly levels of cyanide when eaten must be removed.

VOA: special.2010.06.08

The scientists gave some of them a substance from apples called polyphenol antioxidants.

VOA: special.2011.03.29

He was studying a substance in the hull that covers rice.

VOA: special.2009.04.21

Tar is a substance produced when tobacco leaves are burned.

VOA: special.2010.03.09

The researchers say their study shows a vaginal gel substance reduced the risk of HIV infection among women who used it.

VOA: special.2010.07.21

Scientists at the Goddard Space Flight Center in Greenbelt,Maryland, found the substance glycine in material brought back to Earth from a comet.

VOA: special.2009.09.30

Her experiments for the project were supposed to produce a clear substance similar to a thick syrup.

VOA: special.2010.02.09

Cells in the airways begin to produce too much of a thick, sticky substance called mucous.

VOA: special.2009.07.28