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)