Whether you are soft boiling an egg or hard boiling it, you don't want the shell to crack. Shells tend to crack due to abrupt changes in temperature. For example, taking an egg out of the refrigerator and placing it into boiling water. To keep your egg shells from cracking when they are boiled, you have two options.

First, you canplace the egg in the water prior to heating the water up. This makes your cooking time a little harder to determine, but it will keep your egg shell from cracking.

If you want to control the cooking time more precisely, warm up your eggs before you cook them. Take them out of the refrigerator several hours ahead of time. Put them in some warm water. That way when you add them to the boiling water, there is less of a temperature difference and they are less likely to crack.