With some pens, you can jam the spring between the button and the silver thing on the bottom right and the white thing next to it. Then jam it into the top of the pen with the ink stick until it sticks.
Then press the button on the top of the pen and the white thing shoots across the room.
For us it was strips of paper rolled up super tight and bent across a rubber band. We called them wasps and a well-aimed wasp could give you a sizeable welt.