Joyce Wong, age 12, of Lethbridge, Alberta, Canada, for her question

How big is a living cell?

The dimensions of most living cells are microscopic and the microscope reveals them as flat silhouettes. Their diameters are measured by the micron, which is one 25,000th part of an inch. Some of the smallest living cells are bacteria. The giant members of this world of midgets measure several microns in width. Some of the smaller bacteria measure 100,000 to an inch    which means that their diameters are less than half a micron in width.

The plant and animal kingdoms teem with a countless assortment of living cells. Most of them are but a few microns in width and too small for human eyes to see. However, there are larger living cells that we see almost every day. One is the golden yolk of a hen's egg. The yolks of all bird eggs are single cells    and naturally the biggest belongs to the biggest bird. The largest known living cell is the orange sized yolk inside the egg of an ostrich.