Salmonella bacteria found in eggs prompted a nationwide egg recall of 380 million eggs this week. As the FDA works to investigate the how the outbreak occurred, Life's Little Mysteries has rounded up a dozen of facts you may not know about eggs.

1. An egg shell is made of calcium carbonate, which is also the main ingredient in some antacids. The shell makes up 9-12 percent of an egg's total weight, and contains pores that allow oxygen in and carbon dioxide and moisture out.

2. An egg white is made mainly of a protein called albumen, and also contains niacin (vitamin B3), riboflavin (vitamin B2), chlorine, magnesium, potassium, sodium and sulfur, according to the Iowa Egg Council, an industry group. The white contains about 57 percent of an egg's protein.

3. The color of an egg yolk is determined by hen's diet. The more yellow and orange plant pigments there are in the grain fed to a hen, the more vibrant the color of the yolk will be.

4. The other colors within an egg vary with its age and other factors. Egg whites that are cloudy indicate that the egg is very fresh, according to the Egg Safety Center. Clear egg whites indicate an egg is aging; pink or iridescent egg whites indicate spoilage, and these eggs should not be consumed.

5. The blood sometimes seen in an egg comes from the rupture of small blood vessels in the yolk. It does not indicate the egg is unsafe to eat, according to the Egg Safety Center.

6. Eggs are about 105 degrees Fahrenheit when laid. As they cool, the liquid inside contracts, and an air cell forms between these two layers at the large end of the egg, according to the Iowa Egg Council. You can see the air cell in the flattened end of a peeled, hard-cooked egg.

7. The average hen lays 250 to 270 eggs a year.

8. White eggs are the color preferred in most of the United States, but brown eggs are preferred in New England. A hen's color indicates the color of the eggs it will lay — White leghorn hens, the most common type in the United States, lay white eggs. Rhode Island Red hens lay brown eggs. There are no significant differences between eggs of different colors, according to the American Egg Board, an industry group.

9. Eggs take about 24 to 26 hours to form inside a hen. First, a cell called an ovum develops into the yolk inside an ovary, which is also where Salmonella bacteria may enter the egg. At ovulation, the follicle ruptures, and the yolk is released into a tube called oviduct. On its journey through the tube to the uterus, the albumen is deposited around the yolk, and then membranes form around the albumen. The shell forms in the uterus. Eggs are laid large end first, and about 30 minutes after laying, the process stars anew.

10. About 75 billion eggs are produced in the United States each year, which is about 10 percent of the world's total. Of these, 60 percent are used by consumers, 9 percent are used by the food service industry. The rest are processed and used in products such as mayonnaise, marshmallows and cake mix. China is biggest supplier of eggs, producing about 390 billion each year, about half of the world's supply.

11. Turkeys also lay eggs, but you aren't likely to find them in a grocery store. Turkeys need more nesting room, so housing them is less economical, and they have stronger mothering instincts than chickens, so collecting their eggs difficult. Dinosaurs laid eggs too, and sometimes dinosaur dads were responsible for sitting on them, according to research in the December 2008 issue of the journal Science.

12. In fact, an analysis of another dinosaur egg nest helped solve the age-old riddle of which came first. Canadian researchers reported that eggs came before chickens, because dinosaurs were forming bird-like nests and laying bird-like eggs long before birds (including chickens) evolved from dinosaurs, according to the research published in the journal Palaeontology in 2008.

This article was provided by Life's Little Mysteries, a sister site to LiveScience.