You don't need that much force, but you do have to be sure about it. You know, like the egg you talked about before! If you just do a little tappy-tap, [she demonstrates with the kind of light tapping he'd been doing before, which of course does nothing,] It's not going to do anything. You just need to have conviction and be sure of yourself, but not violent about it! Like...

[She sets the geode on the table and covers it with a cloth to protect from any stone that might splinter, then gives it one, two, three sharp whacks. After the fourth, the geode crumbles, and she takes the cloth off to reveal it split into one large chunk and two smaller, more jagged pieces.]

See?