Why do porcelain light fixture "knocks outs" shatter the entire fixture when struck?

I'm trying to install a simple bare-bulb utility-light made of out porcelain, but twice now I've gone to strike out the knocks outs, once with a screwdriver and once with a hammer and nail and both times I shattered the fixture. How are you supposed to do it. I'm down around $10 now for a darn attic light.