Questions

I like it. Here's some advice.

What is rain?
[s]B[/s]ut tears of the sky(.)
Coming down in empty streams(.)
[s]R[/s]eleasing pain to rip the world,
[s]I[/s]nto scattered, shattered pieces.

What is the heart?
[s]B[/s]ut a broken metronome,
[s]E[/s]ternally ticking down death's arrival.
Red droplets of life within,
[s]C[/s]arelessly lost in one's misery.

You have some sentence fragments in here. And while that's fine in poetry, you want to use real sentences as much as possible. So, look above to where I have parenthesized some periods (.) Those should be commas, and the following capital letters should be lower case. I'd also lowercase those "but"s after the questions. It will help your flow a great deal.

I really like the idea of rain being just tears and comparing a heart to a metronome. What I don't like are the last two lines. They don't make a great deal of sense, and they don't really add to the ending. The previous stanza gave us an idea of what the tears would do--ripping up the world ect. You should parallel that here for the best effect. Plus, a heart as a metronome is unique; blood as red droplets is not.

You can make a better last two lines then that. So, I'd take that same meaning--misery and time ticking down-- and bring it out at the end.

This is a nice short piece. It shows a great deal of potential. Good luck!

What is rain?
But tears of the sky.

I would rephrase it as one sentence.
-> What is rain,
But tears of the sky

good job gigi!!