The heated rock boiling purification system is effective for killing microorganisms if you can build a fire for boiling water, but cannot find a heat-proof container. Instead of using a container, it is rocks that are used to heat the water.
- Fill a container of water
- Filter the water
- Start a fire
- Place stones in the fire
- Once the stones are hot, carefully remove them from the fire and place in the water container
- Repeat the process until the water boils and continues boiling for 5 minutes (longer than boiling over a fire as a precaution)