Pronounced as one letter, And written with three, Two letters there are, And two only in me. I’m double, I’m single, I’m black, blue, and gray, I’m read from both ends, And the same either way. What am I?

I can't be touched, I can't be felt I can't be seen, or heard, or smelt. I lie behind and betwixt the stars, Or hide within the depths of jars, I'm found under the hills, and all the empty holes I eagerly fill, I come early, I follow after, I end life, I kill laughter.

I fly up high with my little eye. For the sky, I am quite the reacher. But I can cause you to let out a sigh when I'm done by a boring teacher.

If you love me, avoid bottles and gifts of fire.

A measuring cup, piano, and most fish have this in common.