Riddle: Such tiny little creatures

We are such little tiny creatures.

All of us have different features.

One of us in glass is set.

One of us you’ll find in jet.

Another you may see in tin.

And the fourth is boxed within.

If the fifth you should pursue, it can never fly from you.

What are we?

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Answer: 

The vowels a, e, i, o, u.