Many people hard-cook eggs by putting them into already-boiling water. The temperature shock, it is claimed, causes the shell to expand quickly and crack, resulting in stringy and unsightly egg white leaking out. But while this theory seems to make sense, it is not true.
First of all, an expanding shell is not likely to crack–a shrinking shell is. In this case, an egg could crack when it is done cooking and is transferred from the boiling water to the cold water bath. Of course, by that time, cracks do not matter.
So, why do some eggs crack sometimes when you boil them? Some eggs already have invisible cracks in them from rough handling during production and shipping. These will indeed open up in hot water and release egg white. In some cases, if the water is actively boiling, the eggs will be bounced around and cause new cracks. Thus, if you must boil eggs, use gently simmering water.
I once saw Julia Child demonstrate this on her TV show. She took a bunch of raw eggs from an ice water bath and put them directly into simmering water, and not one of them cracked. Pretty convincing! This has also been my experience, although Like many other cooks, I prefer the steaming method these days. Please refer to this page for details.