How can 2 liquids exist in a vessel without mixing?

I have a gift article in my hand. It is a small transparent plastic
tube with 2 liquids - one green and one clear. Both co-exist without
mixing. The green sits down while the clear one floats on the top.

When the tube is turned upside-down a beautiful transition effect is
seen wherein the green slowly travels down and the clear travels up.

I want to experiment with such effects. What 'keywords' should I search
on the internet for this ? What line of chemistry is this?

And of course, what were the liquids?

Any other hint/resources/pointers/advise that can get me started would
be very much appreciated.

Thank You,
Mike

** http://mansi.ws - the art of outsourcing **

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