Years ago I read an article about an accidental fire that was started by a fish bowl. I always wanted to try and replicate that event. I bought a small fish bowl and recently uncovered it and decided to give it a try.
Because it was only a few days past Summer Solstice the sun was very high in the sky and couldn’t produce a focal point in a strategic spot. I could have waited several hours for the sun to lower for a better angle but I decided to change the angle of light by using a mirror.
I used some moss and a dry leaf as my tinder but in the fishbowl fire scenario it could be a table top placemat or decoration that catches fire. My particular arrangement had success.
Without manipulation a fire occurrence would be rare indeed but not impossible given the right angle of the sun, a proper focal distance, and dry (probably dark) flammable target material.
Here are some helpful hints when experimenting with water lenses: it is best to stabilize the vessel, the sun angle can be modified using reflectors, use the brightest (and smallest) focal point, and use dry, dark, and compressed fibrous tinder.