Byron Cox, age 16, of Baltimore, Md., for his question:

WHAT IS EMULSION?

Emulsion is a preparation of one liquid evenly distributed in another. Scientists say that one of the liquids is dispersed in the other.

The two liquids do not dissolve in each other. Rather, tiny drops of the dispersed liquid remain suspended in the other liquid. These drops range in size from one tenth to 20 microns. A micron equals one one thousandth of a millimeter.

Some common substances such as cosmetic lotions, foods, lubricants, medicines and paints are emulsions. Photographic film is coated with a light sensitive colloid that is incorrectly called an emulsion.

Emulsions are not permanently stable. The liquids separate from each other after a certain time. To help keep them mixed, an emulsifying agent is needed.

Milk is an emulsion of butterfat in water. The emulsifying agent that keeps butterfat suspended in milk is the protein casein.