Pistachios pop open as they mature. The shell usually splits along a ridge, making opening the shell very simple. Sometimes you find shells that have split open only a little or worse, not at all. This generally means the nut wasn’t mature when it was harvested, but that might not stop you from trying to eat the nut anyway if you’re hungry enough. Try opening the shell with a nutcracker.
Spread a napkin or paper towel out on a counter or table, or place a large plate in front of you.
Put the closed pistachio shell in the nutcracker, turning the shell so that the arms of the nutcracker rest along the ridge where the shell would normally open. Start with the arms of the nutcracker at about the midpoint of the side of the shell.
Squeeze both arms of the nutcracker, slowly. Stop when the shell cracks open.