This summer there was a case that almost led to a fire in the apartment. We filtered the water from the tap, then poured into large glass jars for use on the farm. These glass jars were placed on the windowsill at the window. Once we saw that the towel lying on the table caught fire ... At first we did not understand the reason, but I put my hand to the glass jar in such a way that the sun's rays focused on it and felt the burn. Thus, by chance, the glass jars with water turned into a collecting lens for the rays of the sun, the focus of which coincidentally coincided with the towel and it caught fire. Do not leave any transparent containers with water, lenses and even glasses under the sun in the apartment!