← Fire Making
ConditionalIntermediate · Variable
Water-Filled Bottle Lens
Works only in strong direct sun with a well-rounded bottle or bag. Learn why it often fails.
Question 1
The goal
Concentrate sunlight to ignite tinder.
Question 2
Materials
- Clear round bottle or bag filled with water
- Bright midday sun
- Char cloth or very dark tinder
Question 3
Substitutes
- Magnifying glass (far more reliable)
- Eyeglasses for farsightedness
Question 4
Step by step
- 1
Fill a round clear bottle completely to remove air bubbles.
- 2
Hold it so sunlight passes through the curved section.
- 3
Move it until you find the smallest bright spot on dark tinder.
- 4
Hold perfectly still. Expect it may never get hot enough.
Saved steps go to My Survival Notebook.
Question 5
The science
Curved water bends light like a convex lens, but irregular bottle shapes create a large, low-intensity focal spot.
Question 6
Dangers & limits
- Do not rely on this method — carry two proven ignition sources.
Question 7
How you know it worked
Tinder smokes and glows at the focal point.
My notes
Notes save on this device automatically.