Though it seems fragile, an eggshell is actually really strong because its dome shape. If you have seen the way eggs are sold in the market, you would have noticed that they are kept with their ends pointing up and are never left lying horizontally. Hens, too, incubate their eggs the same way, with the narrower end pointing upwards. What the dome shape of the egg essentially does is that it distributes the weight and the pressure applied on the top evenly to the entire structure, making it one tough nut to crack. Put pressure on the middle part of the egg and it will crack right open.
Check out this experiment done by Imagination Station when a 30lbs cement block is held up by …eggs!