Sunburn caused by water

Solution

The water drops act like a converging lens. So on their surface the incident light is refracted to a point behind the drop. For this reason all of the light, which falls from the sun onto the drop, is bundled in one single point in the extreme case.

Roll your mouse over the picture to see the path of the rays.

Together with the light, the energy, which is transported by the light, is also bundled. The energy, which would normally be spread on an area that is as big as the water drop, is now focussed onto one single point.
There is no additional energy produced. If you look very closely, you cann see darker shaded areas next to the focal points. The light, which would fall upon the leaf there, is refracted by the lens and thereby deflected. Therefore it is a focussing from the area to one point. In this case no energy is generated: As a result there is the shaded area.
By focussing the energy on one point, it is heated significantly more strongly. Thereby the point on the leaf is heated at an above-average rate, so that the leaf can be damaged.

But the path of rays displayed in the picture corresponds to a very specific extreme case. The flowers on the window sill in the first part of the brain-teaser had damaged leaves. But in their case those specific conditions were fulfilled, so that the sun could damage the leaves. However, in case of water drops those conditions are only fulfilled in rare exceptions in opposition to the window in the bath room, as you will see on the next page.