Answer:
I have joy in bringing two together, but darning my existence! My life hangs by a thread, filled with ups, downs and resistance!
Mouth up it gets filled, mouth down it gets empty.
What you do to determine the length of something
Sends paper without using the post office.
Sometimes you feel it weigh a ton. Yet it is nothing, but somehow still there. Makes you uncomfortable around anyone, But speak its name and it's gone.
How can you add eight 6s together so that the total adds up to 750?
Easy
Animal
Difficult
Food
Funny
Kids
Logic
What am I?