Today at our community garden event, during the end of day lunch break, one of the gardeners asked if that was smoke she saw at the end of the tables. Of course we all poo poo'd her. Heck, what could be on fire? It's only a jug of water, and at the time, a few assorted pots of lunch fixins. But sure enough, it was indeed smoke! A tendril of smoke was swirling up from the surface of the card table.
Closer inspection revealed that the glass jug of water was acting as a lens, focusing the sun's rays. Just like in science glass, concentrating the sun's energy on a combustible focal point resulted in ignition.
A good sized hole was melted right through the surface of the table and the plastic was on the verge of erupting into flame. Impressive!
Note to self --- put water jug in the shade for now on.