Invisible Liquid!

It was 28th of February, the National Science Day. Sheela was wondering what could she present on the eve of Science exhibition arranged in her school. With no other thought, she decided to approach professor Govind. No sooner she opened the laboratory, she was puzzled. Professor was inflating some balloons. 

She asked him whether he's turned a baby with sarcastic undertone.He welcomed her and asked what can he do for her.She explained her situation and requested him to guide her with a science fair project.

He asked, "Why shouldn't you do this?"

"This!! Professor, I'm sure that I ain't a baby," shouted Sheela.

Govind said, "Hmm, say that you're familiar with helium balloons".

"Professor, I know that helium is lighter than air and hence a balloon inflated with helium floats to the top; what's new with this??" asked Sheela desperately.

Now professor didn't speak anything and went to the junk box and took an empty glass fish tank. Then he started doing something. "Professor, I was asking about my science project, please", Sheela said. Professor never uttered any words and was keen in his job. Sheela watched what he was doing.

(watch the video)

Sheela
could not speak for a moment."Woww!Invisible liquid" she exclaimed.
"So, how about you doing this for your science fair, Sheela?" asked
professor. 

"This is excellent professor. Explain me how it works", asked Sheela.

"The principle is nothing but what you explained me earlier. Density matters..." smiled Professor.

He
continued,"I just closed this tank with a wooden board and filled it
with Sulphur hexafluoride gas. Sulphur hexafluoride is roughly 5 times
denser than air. Lighter gases tend to displace heavier gases downwards.

Hence SF6 displaces air upwards".

"Thousand thanks for you professor," greeted Sheela.

"But be careful.This gas is much poisonous and also a green house gas", explained Professor.