So, I have a small tribe of island people with stone-age technology who have [discovered how to make gunpowder](https://worldbuilding.stackexchange.com/questions/77880/what-explosives-could-a-small-island-nation-feasibly-discover-without-metal). However, they want to make war on the neighboring islands, and they need a delivery system for their new explosive because blowing holes in your enemy's canoe is fun. They have decided to use the arrows they already use to launch the gunpowder, by packing gunpowder into a hollow stone arrowhead. However, they require some way to set off the gunpowder upon impact. **How does a tribe of stone-age islanders create an impact-trigger for explosive weapons?**