Question

As mentioned, transparent objects do not cast shadows at all. But, if we take a glass or a transparent ruler, and hold it above a paper, tile etc, a faint shadow is cast. Why is it so?

Answer 1

as answered previously.

Shadows are formed when an opaque object is placed in the path of light rays. The rays go past the edges of the object and make a shadow.
This is true that water (or a common ruler) is transparent, but, when the bulk volume of water flows it almost acts as an obstructing object which blocks the ray of light and shadow is formed.