Once that's decided (ha!) we now have to break the shell and get to the yummy, runny yolk. Here are your choices:
Tap. Using an egg spoon (a regular teaspoon will also do) give the top of the egg several sharp taps to crack the shell, then use the tip of the spoon to wiggle through the cracked shell and slice through the egg, lifting the top off as you go. This is the safer approach, although you may encounter bits of egg shell if you're not careful.
Whack. With a butter knife, hit the side of your egg with a quick, decisive whack. This should cut through the shell and most of the egg. Then simply lift the knife, thus lifting the top of the egg, and remove. This approach is a little more daring as too much force could lead to a rather unpleasant mess, while too little will leave you with a version of the tap method.
Combo. Alluded to above, this is simply using the side of a knife to tap around the sides of the egg in order to crack the shell.
Implements. There are several versions of scissor-like implements (pictured above) that cut though the shell and the egg, as well as a popular mini-plunger (pictured below) that will cut through the shell, allowing for a nice clean break. These implements are for posh folk who frequently eat soft boiled eggs, or those who like to acquire unitasker kitchen tools.
As with most techniques, much depends on what you were raised with and therefore feels most familiar. One thing we can all agree on: soft boiled eggs must be served with soldiers (buttered toast cut into strips) for dipping into the runny yolk.
How do you open a soft boiled egg? Big-Endian or Little-Endian? Whack or tap?
Related: How to Soft Boil an Egg