It's a colorless gas with a pungent smell.

Why did we introduce first a drop of water?

HCl dissolves into this water, creating an under pressure.

Explain the fountain!

Chlorhydric gas disappairs while being dissolved in water. The vacuum aspires water.

What is the nature of the colorless liquid introduced in the flask ?

It is a water solution of chlorhydric gas. We call it chlorhydric acid.